I am, but I had to set the SDRAM divisor to 4 instead of 3, which slows down the system RAM compared to using the 266 MHz chip. I can keep the SDRAM wait state at 2T though (insead of 3T). By way of comparison, cachechk shows that the RAM read speed is:

266 MHz with 2T & Div3 = 148 MB/s
300 MHz with 2T & Div4 = 138 MB/s

This kinda stinks, but the overall Quake score is still up when using the 300 MHz chip.

266 MHz with 2T & Div3, Quake = 36.2 fps
300 MHz with 2T & Div4, Quake = 38.0 fps

However, if I drop the RAM to 64 MB, instead of 192 MB, I can run the 300 MHz chip with 2T & Div3, meaning that the SDRAM is running at 100 MHz. In which case:

300 MHz with 2T & Div3 = 166 MB/s
300 MHz with 2T & Div3, Quake = 39.7 fps

For now, I have elected to keep 192 MB and use 300 MHz with 2T & Div4 because I still like to use the computer to view Vogons. NT4 and W98SE seem to be OK with this configuration. But as a final check, I booted to Windows 2000 and received an explorer.exe crash message after boot-up. So this makes me wonder if 300 MHz, 2T, Div4, and 192 MB are entirely stable. Perhaps I should drop the RAM to 128 MB, or say to hell with it and drop it to 64 MB and run the SRAM at 100 MHz (Div3) instead of 75 MHz (Div4).

Did you run it with register enhancements enabled? Hopefully 128MB is the sweet spot size/speed wise.

Yes I did.