Don't forget to factor in the Z4 3.0si.
I wager that the majority of M drivers are incapable of pressing their cars to the limits of the car's capabilities.
So, a 3.0si may do the trick. Ask your hubby to drive one. Note that the 3.0si at 255 hp is smack in the middle of the horsepower range between the 2.5 (184 hp) and the M (330 hp). Its a significant power increase (an extra 71hp) over your current 2.5. And as one 3.0si review says:
"This engine pulls ferociously- especially on the highway where passing power is frankly startling."
So - what more do you want?
Good luck!