I don't think I could bear to look when my car is remapped and put on rollers... even though there is a fan providing air to the radiator, it still doesn't match the draught you get when actually driving on the road and it can't be good for any car to be hammered like that on rollers. 
Its not so bad it isn't flat out for very long at all, just a short shift through the gears then to the red line or there abouts in fourth, no problem.
274 @ the wheels is a hell of a lot!
nice one 
least they couldve done is pump your tyres up to get to 300 lol.
I have no doubt it'll do 300 once loosened up a bit more, it had only done 1600 miles at the time, its done 2000 now and just feels stronger and stronger