Interesting topic!

I think that the main attractive of the K6-2+ is that not only you can change the multiplier but also you can enable/disable individual caches (L1, L2, L3/motherboard's).

Doesn't that give you some granularity between all caches off and Pentium MMX 200MHz?

If you manage to get the speed of a 486 66MHz, that's a sweetspot for early 90's games. Not that most games wouldn't work with 133MHz or 200MHz CPUs, but you can have some problems as you increase speed.

With faster processors, Sierra games fail to detect the sound card, there's the error 200 (though most EXEs can be patched), and the speed issue with some games that run way too fast. Those are the most common problems that I can remember.

For older older games things get trickier. This is perhaps the main difficulty with retro games.
Most processors will give you a slow 386 when you disable both caches.

If you wanted to play let's say Wing Commander II, you would need a fast 386.

Sometimes for really old games you need even slower speeds.
That's when you can use SLOWMO, SLOWDOWN, CPUSPD, SlowDOS, etc.

If you're lucky, you can find a motherboard that lets you reduce the BUS speed below spec.

I experimented a lot with my K6 II+ CPU which can do 2x66Mhz up to 6x600Mhz, with enabling and disabling cache.
As mentioned in the opening post, there is quite a gap of what you can even do with cache disabling.

Basically the Slowest you can get with L1 enabled is a 3D-Bench 1.0c score of 135,7, this equals a Pentium MMX 233Mhz
This is with 133Mhz, Enabled L1 Cache, Disabled L2 and L3 Cache

The fastest you can get with L1 disabled is a 3D-Bench 1.0 score of 43,4, this equals to a 486DX2 66Mhz.
This is with 600Mhz, Disabled L1 Cache, Enabled L2 and L3 Cache (using L1DX of setmul)

So everything in between, like a 486 with 75Mhz, to a Pentium with 166Mhz is completely in a dead zone and out of range.

I will probably list a couple of games later today or tomorrow with a few games I tested. But so far I only found Hi-Octane, where downclocking itself was useful (besides of the error 200 ones)
Wing Commander 1 is even too fast on slowest Clockspeed with all Cache disabled. I didnt check slomo and other software yet. But I guess you can use the software anyways? Is there any downside in using these tools compared to using setmul?

I have been experimenting further with the Slowdown tools and BIOS Settings (with a BUS of 66Mhz and a multiplier of 2).
With also disabling the mainboard cache, Frame Buffer, A20 and setting the Dram Timing to Low I have been able to set the Speed as low as 3DBench 10, which is still too fast for Wing Commander 1 (well, assuming that the timing I am aiming for is correct, hard to find references)

If I apply these additional slowdowns to L1 Enabled, then I can get down to a score of 107 which equals a Pentium 120. So there is also a bit of progress. The gap is then still between 43.4 (486 66Mhz) and 107 (Pentium 120Mhz)

I also tried various slowdown programs. With almost no success:

Throttle: Says my Chipset isn't compatible
DOSSlow: Only brings score down from 11.9 to 11.3, no matter what value is set,
Pentslow: Just hangs the system
Slowdown: Seems to be a bit unreliable, also does not work when EMS is needed.
Slow (Slow 300): Tried several configurations. while DOS processing and DN starting up slower, it doesnt seem to have any impact on 3DBench or Wing Commander. Chris Benchmark totally freaks out.
Bremze: Bremze70 Leads immeadiately to glitches, Bremze08 immidiately turns itself off
MoSlo: Expensive and Commercial, doesn't low down beyond 10.9 (from 11.9)

A few games I tested:

Descent 1:
600Mhz L1 Disabled: In 320x200 game is running slow, but kinda playable, High Res not tested
200Mhz: in 320x200 game is running fine, on 640x480 game is running ok (probably not quite 30 fps)
600Mhz: in 320x200 game is running too fast, on 640x480 game is running fine
Conclusion: If you absolutely wanted to play in 320x200 then downclocking would be useful. If you want to play 640x480 (which was patched in, needs to be started via command prompt -640x480) then it's not necessary.
No Use in downclocking.

Theme Park 1:
133Mhz: Game is running unplayably fast in low Res Mode. In High res mode it works fine, but animations in all management screens are too fast
133Mhz L2 Disabled: Very close to normal 133Mhz, still too fast for Menus
133Mhz L2 Disabled, External Cache Disabled, Frame Buffer Disabled, A20 Disabled, Dram Timing Set to Slow: With this equivalent to a Pentium 100, the High Res Graphics on the main screen are too slow, while at the same time, the menus are still too fast
600Mhz L1 Disabled: Game is running fine in Low Res, too slow on high res
Conclusion: For Low Res, you need to disable cache. For High Res, it might be that it's just not possible to make verything look good. The moment the High Resolution Graphics look fine, the Menus are too fast.
No Use in downclocking for normal resolution. High resolution seems to be screwed no matter what

Hi-Octane:
600Mhz: Game is unplayable fast
200Mhz: On High Res maybe slightly too fast?
133Mhz: Game is running too fast on Low Res, On High Res it's running fine
600Mhz L1 Disabled: Even on Low Res, game is running too slow
Conclusion: Playing on Low Res with the right speed is impossible as it falls in the configuration gap (as Theme Park). If you want to play on high res, then fine-tuning to the right clockspeed makes sense. 133Mhz or 200Mhz seems to be fine, hard to identify the "intended" speed. 600Mhz is definitely too fast.
Downclocking useful

One Must Fall:
600Mhz: Unplayable fast
133Mhz: Unplayable fast
600Mhz L1 Disabled: Running fine
Conclusion: Got to disable cache anyways
No Use in downclocking.

Quarantine:
600Mhz: Absolutely unplayable
133Mhz: Close to playable, too fast
600Mhz L1 Disabled: Running fine, maybe even still too fast
133Mhz L1 Disabled: Running fine, maybe even still too fast
Conclusion: Controls and how you drive seems to feel weird regardless. In any case, you wont be able to play without disabling cache though.
No or not much Use in downclocking.

Wing Commander 1:
133Mhz L1 Disabled: Game unplayable fast
400Mhz L1 Disabled, External Cache Disabled: Game runs a bit too fast
133Mhz L1 Disabled, L2 Disabled, External Cache Disabled: Game runs still about 30% too fast
Conclusion: No matter what, you will need 3rd party tools to slow down the CPU further. With these I had no success either
No Use in downclocking.