First, I kind of agree with the power situation on the Z roadsters. I just drove a Z4 2.5 and 325Ci on the same day, and thought the 325 had more zip to I. I can't understand why. It's totally counterintuitive. But you seem to be experiencing the same effect.
Second, you may be confusing depreciating with depreciated . There is a difference. You took a couple of big hits. First the drive it off the lot hit. The depreciation curve is very steep in the first three years, then flattens out for a few years. Then you got whacked when the Z4 came out. You could probably hold your car for another year or two without getting hurt much more.
I would use this to do something that is somewhat unusual nowadays, save to buy a new car in a year or two. Take what was and would be the car payment money and bank it. After a year or two, that money, plus what you can get for selling the roadster, should pay the majority of a new BMW. If you need to, you could get a short loan to cover the rest, and not get saddled with a big car payment for 4-5 years.