Mine has Draglites on it, but I still have the stock wheels somewhere:
You can get 20 MPG out of the V6 if you keep your foot out of the turbo.
They only made 547 GNXs so they're going to be REALLY expensive. And if you buy one the only way to maintain your investment is to keep it as a garage queen. They weren't that much faster than the GNs and by the time you apply some modern tweaks to a 'regular' GN you've blown the doors off a GNX. The engine in the GNX was the same as the GN. The turbo had a ceramic coated impeller wheel, but it was the same .60 A/R compressor wheel as the GN. It had a different chip and intercooler. The big differences were in the suspension, which helped it produce better 0-60 and 1/4-mile times.
Despite being the most 'common' of the Turbo Regals with 20,000+ produced in 1987, the '87 GN is the most expensive to buy (next to the GNX and some ultra-rare T-type cars). Your best bet is to pick up an '86 or '87 T-type. They have the same exact drivetrain as the GN but you can get them in lots of different colors and with column shifters and bench seats.
I finally talked my brother into joining the Dark Side. He sold his WS6 Trans Am and picked up a rough 1987 Turbo T. In '87 there weren't any "T-types", they were "T"s, with a 307 V8 version and a turbo version, hence the "Turbo T" designation. He hasn't had it long, but he's already running mid-12s with it. They are a blast to drive and rip up the 1/8th mile in a hurry even in stock form (which makes them stoplight terrors).
My GN is absolutely the most fun car I've owned. You definitely won't be disappointed with one of these cars.
Jim