155 and still accelerating before shutting her down on an open Interstate with a few buddies acting as spotters just so I can say that I've seen 150+.
Black 2001 LS1 with Long Tubes, Borlas, cold air intake and a tune.

 

Few months back I topped out 5th at 174 mph. Little taller tires and higher redline or a gear swap and I am pretty sure 200 wouldn't be too hard. Car felt really stable at that speed, had no trouble keeping it under control and straight.

04 ls1 coupe. Long tubes, straight pipe exhaust, vram, bigger throttle body, lowered an inche or so. Nothing too crazy by any means, but pulled it pretty quick, I was impressed.

 

Some awesome speeds here in this thread.
The C5 is my favorite! Check this out!