Lol well in this case TIG is worst because you apply the heat longer something just doing tack welds with MIG would be better for thats why all body shops use MIG's real easy to warp 22 gauge steel with a TIG weld.
I don't advise subs in the trunk of an MR2 the engine vibrations cancel alot out trust me I've tried it. Your best luck is to cut the factory sub/storage boxes and go from there.