There are some decent CLK's about for that kind of money, but you'd want to be very picky as to which one you bought. That's more of a slushy cruiser though, if you wanted the car you wish your MX-5 was then consider a BMW Z4, plenty of which can be had for that price.

That is not a good comparison Jamie - the drive is quite different! I tested out both and found the MX-5 to be far sportier and more responsive than the Z4.

If you mean 'upgrade' OP, then one must assume that you're willing to fork out for the running costs of something a price bracket or two up. I went racing (as a passenger) with a driver in a Lotus Elise, and he suggested that those are pretty close to the MX-5 but just have that next level of aggressiveness and power.

You could look for an Alfa Spider, a Jaguar XK, a Nissan 350Z, or for sheer grunt, a BMW 330Ci M Sport.

Otherwise, like Collos says, you've got the best all-round sporty convertible on the market! As have I. I think my next car after the MX-5 is probably going to be a Toyota GT86 as I'm not willing to maintain and fuel a Boxster.