Don't go by what the Tire Rack tells you is the biggest rims you can put on the Supra. Plenty of peole on the forums are running 17x10's or 18x10's, with a few running 11's out back with no problems. Alot of the fitment will be dependent on the offset you choose. Also if you have coil-overs, it gives you a little more clearance to the inside.(I think?) It did on my son's 240SX he used to have. I think that 18x9/18x10 combo would be just right. Offset for the fronts should be around 30-40mm, and for the rear 35-45mm should work good. If you want a deeper lip, you could always roll the fenders, and go to a more agressive 20-25mm offset. I'm running 18x8 (20mm) on the front, and 18x9.5 (35mm) on the rear, no problems. If you have any friend that have Mustangs, you could test fit the rims on your Supra, good way to find out how those offsets look and perform on the Supra. Most Mustang widths, are at least 8's all the way up to 10.5", with pretty agressive offsets, 20mm-30mm. Tire size will also have to factor in to the equation also. If you go with say, an 18x10 (42mm) you would be able to run a 285/30, or a 295/30 on the rim.
Hope this helps you;
Tom