Fastest sustained top speeds have been: 150+ mph (1994 LT-1 Camaro). Foot buried into the floorboard for about 10-15 minutes. When I had to slow down, speed did not drop much below 110-115 mph (curves, cresting hills etc) Total distance would be about 85 miles. Doesn't sound like a long time, but perception changes above about 120 mph.
135 (electronically limited) mph (1999 CLK). Same piece of road. Same amount of time (roughly)
Fastest speed (car) was about 300 km/h (about 187 mph) in a modified Acura NSX. That was a rush. The car was very solid at that pace, but distant objects appear awfully fast !

The roads were well known and in very good shape (we have some long straight divided hiways on the praries). Still a pretty dumb thing to be doing.... you never know when Bambi is going to jump out on the road, or Grandma appears out of nowhere. It really is best to leave the insane velocites to the racetrack, or if you are lucky the Autobahn. Even if you have top notch equipment, and are a skilled driver, you don't have much margin for error at 150 mph.
Just my 2c.