All my games need an NTSC or F18A console to run at the intended speed because I'm using vsync for timing. Sprite collision, however, still works on a PAL console. I'm using an NTSC console myself with a European power supply.
I don't suppose there is any simple way of running a pal system at the same speed as an NTSC system, like a timing crystal change? I'm guessing you'd need to replace the VDP also.
One note on using an NTSC console for this--you don't really need to use an additional transformer. Just plug your European external power supply into the console, as the output from it is the same as the output from the US version--it already does the necessary step down for you. The console's internal power supply is the same in both versions.
I have a feeling that it would be better to use an NTSC console with a UK PSU as the games would all run full speed or is this not the case?