Yeah, the calipers are (that's what I have done), but are the caliper carriers?
I don't think they are. IIRC the bolt spacing on the hub is slightly different. A Supra 4-pot caliper bolts straight on to a GS, but you need off-centre inserts to bolt it to an IS.
GS300 Mk1, GS300 Mk2, Soarer TT, SC430, and "J-spec" Supra Mk4 front calipers and carriers are interchangeable.
GS300 Mk1, Soarer TT, and "J-spec" Supra Mk4 rear calipers and carriers are interchangeable.