i don't think there's enough difference for the extra cost of a new Z. besides, I like the one I've got... it's fast enough, i've had it since new, and I think that by tarting this one up (new seats, front lip, possibly side skirts, popcharger, new nob...) that would make me happiest...
but then I read about the new supercharged elise in the sunday times... clarkson wasn't a huge fan so it must be great!