Most rolling roads have a limit for maximum speed - it's becase of the balacing of the rollers etc. For example the one we have at 4Turbo is limited to 220kph (from memory). It does not mean it won't work over that speed but it isn't entirely safe to do that. Still it's not a problem as you can map the car in 4th or even 3rd with enough load to simulate driving to the top of 45 degree slope in 5th flat out.

The thing is you will still want to check the performance and data in real world. Sometimes it can hit some peculiar point in the map only on one track, road or in one situation and give a problem. We had this with one Evo 9 which was getting knock on road fuel only on one particular circuit and this was not possible to replicate on rollling-road or highway. We never got opportunity to track it down completly but it seems it was due to immense heat soak on the track on a hot day with several consecutive fast laps.