Historically the AA S400/500 actions have given very flat power curves over a very useful number of shots without regs.
So if you are getting a 30 or 40 shot flat spot that is giving you under 20fps ( you'll probably get much better than that ) variation over those shots then a reg is probably not required. Some after market regs will eat into cylinder space however they can make better use of the available air so can still give a high number of shots. They are, of course, something else to go wrong.
Personally, having just bought it I'd be doing some chrono string tests and accuracy tests and if the gun performs well, and probably beyond the shooter's ability, which it probably will, I wouldn't worry about it.
Last edited by skires; 12th October 2015 at 12:19 PM.