I boosted the Vcore at 3,66V (by adding 220k resistor at JP46) and was able to run at 160MHz . I don't know why but I have to increase L2 cache WS from 0 to 1 to be able to boot otherwise it always hangs at "Starting MS-DOS..." message (slow down of ISA and IDE related options doesn't help). On my DataExpert EXP4044 the L2 cache works fine with 0WS, the same 15ns Winbond chips...
Despite this handicap I broke through 50FPS in Doom and near to 16FPS in Quake:

MB CPU fCPU fFSB Speedsys CPU/MemBW VESATEST Doom* Quake 320/640**
Hippo 10 (WT) AMD Am5x86-P75 160 MHz 40 MHz CPU: 51,58 / 107,80 MB/s 15 MB/s 47,16 FPS 14,8 / 6,1 FPS
Hippo 10 (WB, L2-) AMD Am5x86-P75 160 MHz 40 MHz CPU: 59,72 / 118,16 MB/s 16 MB/s 50,38 FPS 15,4 / 6,6 FPS
Hippo 10 (WB) AMD Am5x86-P75 160 MHz 40 MHz CPU: 59,71 / 107,80 MB/s 15 MB/s 53,82 FPS 15,8 / 6,7 FPS

BTW it can be seen L1 has much more impact on the performance than L2. So I guess when I upgrade to 512kB (if that chinese crap from Ali will work) it wouldn't help much (just help to cache bigger RAM)

The larger L2 cache will have negligible impact, in my opinion. At least for somewhat period correct software. If you find a way how to turn L2 cache to WT mode, I think you'll break the 16 fps barrier in Quake.

Maybe, I had a look at that ECS MB you posted, it should use the same main chipset and it has L2 WB/WT option. I'll try to find some flash chip and burn the BIOS. If we at least know how to access chipset registers it could be possible to find some bit that controls WB/WT and make some utility that will toggle it using regular Octek BIOS. I also should try to look via Modbin if there are no some hidden options...

I tried to unlock anything with the modbin, with no avail. Hopefully the Phoenix BIOS will work.

Aha, OK.

I just tried to flash ECS Phoenix BIOS but it behaves so bad on this MB. Yes, it has options for L1 and L2 WB/WT but HDD autodetection doesn't work, COM/LPT ports report IO address conflict, turbo switch halts the system instead of slowdown, Doom runs significantly slow and Speesys hangs at "Determining system contents... Memory type" so I cannot compare it. Checkit reports it's ~10x slower than 486DX4 - 4911 dryhstones. Just for completness I tested Doom with L1=WB, L2=WB - 3,81 FPS, L1=WB, L2=WT - 3,81, L1=WT, L2=WT - 3,81 FPS (tha same). So this BIOS is useless.

Too bad, I really hoped it would work. I guess the only thing that can be done is finding an ECS board in which this BIOS work, finding out which bit are changed when selecting L2 cache mode and make a TSR out of it, or perhaps modify some Award BIOS.