On the 7MGTE, it only moves the power around from what I've seen.
The gains are pretty expensive compared to more boost pressure and fuel.
I'd spend the money on larger valves, some port work, and correctly adjusted settings for everything.
Key points are to leave a small anti-reversion lip on the exhaust ports, about 1mm to 1.5mm is enough, especially if you port the exhaust manifold some to add another lip there too. (Think venturi going into the manifold.)
On the intake, either remove the valve guide posts, or smooth/blend them into wing shapes. On the exhaust, remove them so the runner is round.
CompCam's valve springs are an easy, and cost effective mod regardless of if you use stock valves, or larger ones.
Most of all, have fun with the Supra. They are great sports cars!