In southern Germany this morning, I managed to get my NSX up to 299.3 km/h (186.0 mph).
After rounding a bend on the Autobahn, I saw a long straight in front of me so I floored it. In the middle of the straight I caught up with another car and slowed a little while overtaking. Once safely past, I floored it again and kept it floored until I needed to brake for another bend. By the end of the straight, I had gotten up to 299 km/h. The car was barely accelerating anymore, but if the straight would have been longer, if I hadn't slowed while overtaking that car, if it would have been cooler out (OK, enough excuses), the GPS would probably have shown just a bit over 300 km/h. Maybe another day.
I was in fifth gear and at that speed, the tachometer was pretty much at 8000 rpm, similar to where it was when
this picture was taken. There was no whistling from the windows, there were no vibrations, and the car felt very stable (the absense of wind doubtlessly helped here).
My car has a complete front undertray, which is the same size as that on a 2002 NSX-R, don't have a vented hood, and the coolant temperature gauge never budged from its usual position just below the halfway mark. Driving through city traffic on a hot day, the coolant temperature never changes, either. Maybe you would need a vented hood if the engine is running at full load a lot but you're not going fast enough to get decent airflow. Maybe this could happen on tight tracks. However, in city driving and on the Autobahn, you don't seem to need a vented hood even if you have a full front undertray.
For the record, I timed my car using my Palm Pilot, a bluetooth GPS receiver, and Cetus GPS software.