I think the highest powered naturally aspirated S54s are at around 380 crank horsepower, right? I'm not counting the PTG cars which will probably need a rebuild after 10K miles or less and I believe they are not using a stock shortblock anyway (higher compression). So that's around 15% or so. Figure the same for the S65, and you get about 60hp after all possible bolt ons. Don't expect much more torque though (maybe 20 ft-lb).
If you want more than that, you need to go FI (and thus spend big bucks).
A gen-u-ine BMW eff-eight-zero with them tandem clutches in the transmission and that dad gum sun roof on the top-a da cawr.