I replaced my super low milleage TEMS with coilovers and never looked back.
It all depends on what your tastes are and what you intend to do with your car. For me I don't care much for comfort and I like a super stiff ride with minimal body roll. Plus I only drive it 1500 miles a year.
If you daily drive your supra I'd recommend sticking with TEMS.
If your TEMS are shot and you want to retain the TEMS feature your only choice are tokico ilumina 2 struts and some type of lowering spring. Your other low cost alternative that would yield increased performance and still maintain comfort would be bilstein struts with your stock springs. (You can lower your ride height with these too)
As for the Tems, I could feel the difference when the system stiffened under sharp turns, heavy braking/acceleration. Its a nice system that offers good performance with comfort. Contrary, it is easy to exceed the limitations of the TEMS if you push them hard around corners. Once I went coilovers, the performance aspect went way up but comfort levels went down the toilet on anything but decent roads. As I stated earlier, it all depends on your tastes and what your looking to do with your supra.