P-990-- I was told in order to stabalize any thing over 70 gns . You need at least 2800/FPs muzzel to hold it out past 400 yards.
Quite frankly whoever told you that is full of it. And ignoring the body of experience of THOUSANDS of Highpower shooters.
You only need to drive 75-80gr bullets to 2600-2750 fps from a .223 to get reliable performance out to 600 yards. You need a 1-8 or 1-7 twist for this; I could see somebody using a 1-9 twist trying to compensate for the slow spin with more MV, but that's a dangerous game to play.
People who aren't experienced or confident doping wind will also try to use increased MV to compensate. Again, a little practice in the wind will be more helpful than throwing your bullets faster.