I haven't tried myself but I remember reading in the past that Peg1&Peg2 accept different speeds. IIRC Peg1 accepts up to 666Mhz and if you try to overclock it more it will "underclock" to 400Mhz or something like that. In contrast Peg2 allows up to 800Mhz. There was an user in pegasos-forum.de who used an overclocked Peg2/G3 (can't remember his username, sorry) he posted pictures about ArtEffect running on MorphOS if that helps to find him nd ask him.
I did test to overclock to 800mhz but that must have been too much, at first it seemed to work but morphos would not boot, so i tested 733mhz instead and so far it's working nicely :) I'm very tempted to overclock the G4 now also but since i dont have any spare G4 module lying around i will not risk it ;)