So lowering springs suck. Rates are all over the place, the balance is usually off, most are progressive and who knows what the actual rates are for them. That doesn't mean that someone should post a link to the rates post in the suspension section, it means that no one knows what the force displacement curves look like for any of the progressive springs.
So now that we've gotten that out of the way, I dont know why people are so scared of spending money on their suspensions. A supra with a well setup suspension is so good. It makes the car not feel like a giant floating piece of crap. I really wish everyone could drive/ride in a supra with a good suspension on it and then you'll realize what you're missing.
As for asian coilovers, hasn't this all been discussed before? In general, asian dampers suck, poor damping curves, wrong damping ratios, adjustability that doesn't adjust, they're just pretty crappy. Also spring rates on a lot of these setups are kind of all over the place. Some of them are alright, some of them seem way off.
But they're available, and despite the downsides of those coilovers, the cars will handle better than with lowering springs and crap like GR2's or any other OE replacement damper other than a Bilstein or Koni.
But there are tons of options for this car. I make and sell mounting that can be used to mount any damper with 1/2" eyes. So pretty much any motorsports damper can used on the supra.
The formula is simple, good damper, good springs, awesome car. A good damper can be valved for anything you want. The only thing you need to decide is what you want the car to do, how you want it to sit and how not cheap you want to be. I see people putting tons of money into the engine bay and running around on lowering springs and sawblades and it breaks my heart. I die a little inside everytime I see something polished in the engine bay and 20 year old dampers.