The built in signs in TRS2004 go up to 170mph. I believe 06 probably has the same signs. Check under USA Default.
On the DLS, I'm not sure. I picked up some invisible signs and the TGV signs, but I'm not sure what the top speed is.
Try doing a title search for mph and limit the results to trackside objects.
Good luck.
I found some signs in TRS06 labeled "Speed 160" and "Tside Speed 180", but I don't know if they are mph or metric. You could put a couple along the track and run a locomotive past them in quick-drive to see what the imperial measure says the speed limit is.
I did a definitive test.
When passing the Tside Speed 180 sign the speed limit becomes 180mph. When passing the Speed 160 sign, the speed limit becomes 99 mph. So the Tside Speed signs are imperial and the Speed signs are metric.
So there are your high speed signs.
I do wish the content creators would specify mph or kph when they name the signs. It would make it easier to match the prototype standards.