A friend and I did some comparison shooting of his P99 and my M40 both in .40
I found that some ammo he bought in bulk, Magtech 180g I believe it was called, managed to shrink my shot groups to close to half the size of what I shot with all the main target brands. M40 owners, cheap sellor-ballot ammo would not fire in my M40, poor seating and primer only hit hard enough to scratch some red ink-like substance on the back of each round, which resulted in 50 rounds for the P99.
My post though was about a comparison other than the sights and grips which seem to predominate the talk between M40 and P99.
1. P99 mag release requires second hand or change from traditional technique to utilize your first or 2nd finger on the opposite side.
2. Trigger pull, the different button on the top of the slide which changes the type of trigger pull is unusual to me in my experience, Needs second hand and I prefer a normal decocer/safety like Berrettas have.
3. Tried Hogue slip on grip on my M40 (P99 has vastly superior grips for numerous reasons) The model recommended by Hogue was the 17000 which is for large Glocks. It had poor fit, the extra finger groove was in the middle of a digit and when pulled up to align with the factory grips groove it then was loose at the top and had minimal coverage in the front of the grip. I would wait to buy one until one is specifically made to fit.
Any military types who have nomex flight gloves would find that they have good grip on polymer frame handguns.
Happy Shooting