Do you have a IIsi? Have you tried overclocking it by replacing the 40.0000MHz oscillator at G1 with a faster one? Then let's collect our successes and failures here. I'll go first.

I have a IIsi (well, four actually, but three are still broken) that I have successfully overclocked to 25MHz by using part number MXO45T-3C-50M0000 from CTS Electronic Components. This is a 50.0000MHz 14-pin DIP oscillator rated for 5V operation and TTL output. Since I had the board out for repair, I opted to desolder the original 40MHz oscillator and directly install the 50MHz one. (I opted to not use a socket as I haven't found one I like for oscillators.)

Why do I ask? Rumor has it that Apple designed the IIsi to run at 25MHz, then changed it to 20MHz before shipping so that the IIsi didn't take away sales of the much more expensive IIci. Low End Mac even mentions that some users have been able to upgrade their IIsi to 28MHz (which would require a 56MHz oscillator). I thought that if we shared our successes, others might try to upgrade their IIsi to make it a bit less crummy.

It works, I've done several, however you are now running also the PDS slot at 25MHz, and not all expansion cards are happy with that. Most of them work just fine though. The Interware Booster will for example not work since it would mean trying to run the CPU at 75 Mhz, I have also had some issues with my fpga boards and the Carrera 040, but I guess those are all special cases anyway.

As for performance there is a noticeble improvement, especially for the graphics, it's now much snappier.

So in general, this is a safe uprade.