In that particular supra's case there can be other things that could be wrong like a faulty fuel pump. In a supra I would've wanna run the stock fuel pump anyway. Our cars have a lot better designed fuel pumps. But Zs also develop problems with fuel pumps as they get older. You just have to make sure that everything is in good working order.
I've seen a supra on stock fuel put down 540rwhp. He upgraded the fuel system soon after that though.