Well, I have a 2005 2.5 and am very happy with it, you know when it comes to power, how often do you really use it to the max, how often are you redlining your tach. Really, if it is all the time then go for the bigger engine and pay to play price, when I see tests between one car and another and one car goes 6.0 or 5.9 and the other is 5.8 and 5.7, who really cares, well to some it makes a big difference, but in the real world and not on paper, were talking pennies in our pocket. PLus having the bigger engine means your burning more gas all the time no matter how hard you push your car and so forth.........I would not give up my leather,hid lights, auto dimming mirrors, etc......for a bit more push when I put the pedal to the floor. Relax enjoy the ride. The 2.5 has more than enough power and is super smoooooth to be very fun. Imagine driving a mustang gt? the feeling of the fast power in a straight line will fade quickly. The z4 is a fine tool like a swiss watch, sure you can buy the M but for 20-30k more is is really worth it? if money is no issue at all.......then ok do it, but it won't make such a huge difference anyway......wanna laugh.....I put a M decal on the back of my car and it looks great, aside from the double exaust from the back you can't tell. Hope my car does not get stolen because it is marked M that would be the kicker.
Have fun and enjoy your ride.