There is a bit of variability from what many people say an unregulated gun will do in terms of shot count and spread.
My daughters 400 would do 40 good shots within an 8fps band with weighed pellets, plus another 15 if you opened the tolerance up from 8 to 15fps. You have to be careful to put the right fill pressure in, then make sure you don't shoot too many.
In regulated format, the gun would do 100 shots, 7 fps total spread with the same weighed pellets. The pressure gauge on the 400 doesn't read cylendar pressure, but instead reads the smaller regulated chamber pressure (approx 90). The regulated gun could be filled to 200bar and shot down to reg pressure, so a much larger working band than without reg.
The best aftermarket regulators currently on the market IMHO are Dave Welhams (airmasters88). I've had 4 of Dave's regs over the years and recommend his work highly.