I drove a Z4M convertible a few times and decided that the convertible experience isn't for me personally. When I'm out driving for pleasure, I'm focused on the road and don't really have time to enjoy the view above, behind, and beside me, so the main benefit for me was moot, and I'd be putting up with extra weight, lesser handling, poorer aesthetics, getting sunburned, and paying more for the privilege. That said, I did order a moonroof with my coupe since I do appreciate the extra light and engine noise that opening it affords, and moonroof open plus windows down gives me just the amount of convertible experience I want with almost no downside except a little extra weight.
I agree with the instructors that you *probably* won't feel much of a difference on public roads. However, when I add an extra ~400 pounds to my car by carrying 2-3 passengers, I can definitely feel the difference in both acceleration and agility. It's not enough to piss me off, but it definitely reminds me that I can't drive my car the way I'm used to on back roads while I'm carrying them (the other reminder of that comes from their screams
). But I'm also a track junkie and as a result I've become more sensitive to sensations like that, so depending on your driving style you may not notice at all.
But on a tangent subject, what's up with the "especially if I don't track"?? Take your M3 to the track, dude! Even if you only do it once to cross it off your bucket list, there's absolutely no public road substitute for the track experience and no other way to truly discover the M3's purpose in this world.