The N52 is dead reliable, but not much fun in the 5-series, barely any fun in the 3-series (I had an E90 328i). About the only real modification is EAS' supercharger kit.
And while the M6 Gran Coupe is nice, the 535i is still the better deal for power and, dare I say, more reliable. $25k can get you a well equipped and under 30,000 mile E60 535i with the N54 engine. Add another $8,300 to an otherwise stock car and...:
Yes, 829 RWHP and 781 ft-lbs on stock internals and you can have it with a wagon!