There was some reason it had to be on the road; the datalogger used GPS to get its signal I think, hence it had to be moving rather than just wheel speed.
157 isn't redline in top in an RS6. Redline in 5th is the far side of 200 (205 rings a bell) though as standard they are officially limited to 155 with most of the magazines suggesting they were allowed to go a bit quicker than that. After remapping they are derestricted, I saw just under 180 in mine and it was still pulling but I could also see traffic ahead and the rubber seals round the doors all popped at about 165 so it seemed like that was enough. They are all 5 speed autos BTW, so no 6th gear.