Phil
I think the GS300 pads are the same SIZE as the Jap Spec Supra TT, The original Toyota GS300 pads would probably not be the same material (softer/quieter?) as the J-spec Supra TT, but if you buy after market ones then they should be the same material whatever car they are for (e.g. Poerterfield R4S pads are all the same material for every car, which is why the Type R stops so quick pads are bigger than J-spec Supra ones).
Dean
I have some brand new TRD 9270S fast road pads (Supra 93.5~ JZA80(16inch), Soarer 91.5~ , Aristo 91.10~, Altezza 98.10~, Mark II/Chaser/Cresta 92.10~, Celsior 92.8~94.9 ) in the garage if you are interested. I can send them to you if you want to check out the size.