Setting aside the issue with not being able to floppy boot if a 286 CPU is installed, I moved onto the SuperChip's surgery. I was able to remove the SLC-25 with hot air and no physical damage. Here is an image of the PCB traces in case anyone is looking for it at some point.
I applied some TS391AX solder paste as follows,
First time was a charm in this case - no bridges, no mess, no non-contacts.
Here's the setup on the 286 board,
The good news is that the SXLC works on the SuperChip486. The bad news is that when I go to inhibit the cache region, it freezes,
I also tried just using cyrix -i1, but the result was the same - hang. I then tried to first disable L1 with cyrix.exe -e-, then to inhibit the cacheable range with cyrix.exe -i1, but with this order of operation, the whole screen turns black. A real bugger. Deciding to leave the L1 disabled, I then tried to enable clock doubling with cyrix.exe -cd. The L1 mode correctly goes from 2-way to direct-mapped, which indicates that the command registered, but clock doubling did not enable. cachechk.exe indicates the CPU frequency is still at 25 MHz and Landmark's speed test shows the same benchmark before and after trying to enable clock doubling.
I've run into one 386 board which didn't let me enable clock doubling if the FSB was too high, so perhaps there's a similar issue here. I did try to run the FSB at 8 MHz instead of 12.5 MHz, but clock double still did not enable. I guess the SXLC doesn't like the external 2x multiplier on the SuperChip.
I was able to enable L1 just fine with the SLC with cyrix.exe -i1. Perhaps the PAL chips, which were programmed with 1 KB of L1 cache in mind, don't work if there is 8 KB. Since I don't have the DIP switch settings, I am unable to determine if there is a DIP switch which is for use with an SXLC, but I highly doubt there is such a setting. I suppose I could randomly try DIP switch settings, but I doubt there will be any improvement
Out of curiosity, I tried to run the SLC and SXLC with a 13.81 MHz and a 16.25 MHz FSB, but the screen stayed blank at power-up. I guess the PAL PLL can't handle that freq.
Unless someone else has a recommendation, I'll have to solder the SLC back on.