Hmmmmm.....
Assuming stock transmission and rear end gear ratios, I would think that even a mild TR (mabey 300 rwhp) could go 140-145 given a long stretch of road. As for the real strong ones (450+ at the wheels), it would run out of gear pretty fast at about 150-155, assuming a power peak of about 5500 rpm. I am hardly and expert, so if anyone can correct me, please do.
I have had my 87 T up to 4400 rpm in top gear, which should be about 140 mph. It was on a straight stretch of road with minimal traffic, ( Rt 380 in PA towards Gouldsboro/Tobyhana if anyone cares), and I backed off because, well, a Buick Regal at 140 is crazy, but it would have kept pulling. It got to that speed quite quickly. At the time, the car had a totally stock engine with a TA60 turbo, V4 intercooler, 009 injectors, ATR 2 1/2" exhaust, Terry Houston downpipe with a cat, Mark H cold air kit, and a chip by Jim Testa. I was running 17 pounds of boost on Sunoco 94.]
So you see, these cars are limited by rpm, horsepower is usually a non-issue, because even a mildly modded one can easily pull to the max speed of the engine.