I would not do it - I do not find the reduction in muzzle flip to be that much - And, U will have a bright flash when U fire in a dark situation. Plus, in a self defense situation, U might find yourself shooting w/ the gun near your body/face - U are asking for trouble. Another thing - U will loose a little velocity, and it will be much louder.
I prev had a comped 1911 - it was usually the loudest gun on the range. If I ever had to use that gun in an indoor self defense scenario, I would have had some serious permanent hearing loss
And, it might affect your warranty.
I guess if you have an extra P99 and want to make it a range toy - go for it - maybe hard chrome the slide and barrel afterwards. But I would not do it to a carry gun.
If U REALLY have to have a comp, try to see if U can get a longer, after-market barrel and have just the barrel ported. That way U don't affect the rest of the gun.