Thought I'd update folks on the 3.0si now that I've lived with it for a while. In short, I still love the car. And I don't hit my head nearly as often anymore when getting in and out.
I had it up on the Tail of the Dragon last year, with a bunch of other BMW Z cars. Wow! I was extremely impressed with how well the front of the car simply goes where it's pointed. I'm also familiar enough with the handling now that I feel comfortable turning off the DSC, which prevents the nannies from stepping in quite so early. I was able to hang the tail out a bit. The relatively slow corners on the Tail suit the car perfectly.
I also, in a moment of weakness, found a stablemate. I know, I know, I said I didn't want the M. But it turns out I did.
I really like both cars. They have distinctly different characters. The silver car is noticeably lighter, almost 200 lbs. in fact and that is almost all motor. The N52 has a magnesium block, the M car's S54 block is cast iron. So the 3.0si feels more nimble, more eager to turn. And it really doesn't feel like it loses that much in terms of power, even though the M has ~80 extra ponies.
The M car is more, I dunno, 'visceral' I guess. It just wants to GO. More torque at low RPMs, better steering feel (hydraulic vs. electric), lots and lots of brakes (although the si has great brakes too), LSD for when things get interesting. And the color. And, well, it's an M.
That said, the budget does not smile upon me keeping both cars, so the silver one will be going on the block soon. I want to get it spiffed up and perhaps do a couple preventative maintenance items on it first though. And I still want to drive it a few more times. It's a delightful car.