Looks like some tough decisions, but at least it looks like you've thought it out. There are a couple of things that stand out, though.
-The car is paid for and it's depreciating more than you'd like. I think that's a sign that you should keep it. It's in good shape, still a convertible, and if you really wanted, you could always put a blower in it if you [i]really[/] need more juice. Even if it's depreciating quikckly now, BMWs, especially ones that aren't a dime-a-dozen (E36 sedans, E34 525s, anything with an eta engine) seem to have a lower plateau. which the car will nearly always be worth.
-Buy a 2nd (used?) car as the safe/practical/daily driver. Buy what? dunno. Depends on what you need I guess. Compact pickup? Small wagon? (Impreza, A4)