Didn't spotted that thread, but looks like we have pure exhaust-porn going on here
It seems to me for racing use the Akra is one of the best choices judging from official BMW racing team that have all and the best know-how with the HP2, Simo Kirssi's bike:
Altough i'm guessing it has custom made Akrapovich from the factory, probably some twice the lighter than the one they're selling to regular consumers altough it has the same outer appearance - good advertisment method like most of the racing stuff is. The racing scene is for selling more bikes, parts and aftermarket stuff aferall.
The same goes to full BM Motorrad Factory Team's HP2 bike - i'm guessing the crank case, shafts, gearbox (Getrag is their official sponsor btw) etc are probably made different, more expensive, stronger and lighter materials used, lot of expensive handwork invested into bits and tuning (Factory team has sufficent €€€ resource for it, like making the factory R900RR for Dakar rallye costed around 50,000€ or more per bike

), leaving appearance the same but probably the whole bike is around 5-15kg lighter, more power, better suspension and fully configured/tuned for Simo's individual racing spec, than the HP2 they're selling to regular consumers.
For consumer user not racing it in the World class, there's absolutely no difference - the beast has more than enough power with the stock one, not counting the exhaust