I've never ridden a 125 GP bike at the local track here, but an example of lap times:
250SP - 1:07.4 (My lap record on Yamaha TZR250SPR & Dunlop GPr70's)
150SS - 1:08.? (NSR150SP highly modded & Bridgestone treadies)
125GP - 1:06.? (RS125 and Honda Thailand rider)
125GP - 1:04.? (RS125 A kit Thai guy call Film who kicks ass in Jap Championship.
250GP - 1:02.? (RS250 A kit, same guy as above who will probably win Jap Championship this year!)
600SS - 1:7.0 (My time after 10 laps on '99 R6 in race spec, Michelin Pilots)
Supers - 1:06.4 (R1 making about 175hp)
So....depending on the track, absolute lap times may favour a big or a small bike.
A well prepped RS125 is as fast or faster down the chute than a 250SP machine making about 20 more ponies. Weight is a big factor here as well as the GP bike cornering a hell of a lot faster.
The Thai guy I mentioned above has been racing in the Jap Championship for the last couple of years, first in the 125 class and last year in the 250's. He's 5 seconds a lap faster than my 250SP time because:
1) RS250
2) A kit on above
3) Slick tyres
4) HRC support
5) 17 years old so still completely insane
6) He's better than me
Of course, #6 above is very often overlooked, especially when mixed with #1~5