It's a wife car, in all likelihood, she won't care about 0-60, handling, stopping or how good it is compared to 'x'. Anything discussed in this thread would be more than enough for her and whilst I wouldn't buy a 2.7 Boxter, it will feel like an F1 car to her. My missus thinks the MX5 is fast, I find it frustrating on acceleration. An MR2-S would be lovely, providing you can deal with the lack of storage and will be newer and cheaper to maintain than the Porka. The BMW is a better car, but is more of a blokes car than a wife-mobile.
I may be wrong on this front, but the OP's wife wants a Porsche convertible and that is what the OP should buy her, regardless is its the best or not. If she gets the BMW, she will pick up every niggle and complain about it and how it doesn't look as nice as the Porka.