Lead in fuel not only helped with wear on valves in 4 strokes back in the days of 4* (and 5*), it also raised the octane and thus decreased the chance of detonation, allowed for higher compression and more advanced timing, all good for both power and reliability.

Some people here have seen some very good results with the NSRs running tuned engines on unleaded 97 RON (84 MON) pump fuel, though I've read you'll have trouble finding stuff even that good on the forecourt in the US.
However, if the extra octane is there and you tune your engine to use it you'll see more power.

100LL Avgas has a RON of 116 and a MON of 102 (thanks to Stu Jones, works at BP for the RON figure for Avgas) and around 5 times more lead than 4* ever had. It is "low lead" only by the standard of 100/130 Avgas and Bluegas (old 80's/90's leaded race fuel, who remembers GP mechanics wearing breathing apparatus?).
50%/50% Avgas and pump unleaded mix is considered (rather optimistically) by the ACU to be 102 RON which is spot on the limit for a race bike here.

There is always talk of whether Avgas is a good cheap race fuel when it's use is discussed and I don't know enough to really comment with much authority.
What I do know is plenty of people with 250 proddy engines making very good power are using both neat Avgas and 50/50 Avgas/unleaded and plenty of others are running it without problem in place of proper leaded (and very expensive) race fuel in GP 250's running pre 1999 setups.
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