Load of crap, like pretty much all BMW based magazines. There's so much crap in Performance BMW and the like it's not funny. Any car in it not from Scandinavia or Europe you can pretty much ignore as far as their performance claims or power figures go.
Bit of a shame really, the local markets full of a small number of workshops some of which do really good quality custom fabrication and others that do really shoddy work. Unfortunantly most BMW owners who have things done don't have a point of comparision to compare the quality of work to so think it's the norm.
I think generally most BMW drivers are uninformed when it comes to the performance side of car enthusiasm and as such the magazine is skewed towards that by being full of its own self importance and trumpet blowing as you mention.