Normal GP32's can in some circumstances run at 166mhz.
The Turbo Charged ones are just guaranteed to do so. If you buy a "normal" GP32, it may still be able to do 166mhz, but it also may not.
My Gp32 can do 166 no problem, but others cant. A Turbo charged purchase guarantees
166mhz.
I dont know the exact percentage of "normal" GP's which can reach 166, but there was a poll some time back so maybe try and find it. (search on this board).
IMO overclocking is needed to play SNES well, but that may change in the future. Overclocking is also needed imo for Genesis, some PC Engine games, and extra Doom Wads.(also good with GPCinema) Whether piece of mind on overclocking is worth £20 really depends on what you plan to use your GP for.
Finally, if you buy a normal GP32 and find it cannot do 166mhz, all is not lost. It may still be able to overclock to 156mhz or whatever...... it will usually have a machine limit.
I got my GP32 way before the existence of "Turbo Charged GP32's", and got lucky. I don't know what i would have done is Turbo Charged ones had been available at the time, so i cannot really advise you. Hopefully though you may find it easier to make your mind up now.