The optimum wheel size should be what the car came with, because that's how it was designed. You can't directly compare the wheel sizing on the Crossfire to those of the SLK since the suspension is quite different in the two cars. The Crossifre makes use of modified CLK front and S-Class rear suspension to accomodate the larger wheels.

I moved from a '99 Miata to the Crossfire back in July and I've never looked back. They are very different cars for sure, with the Miata being a true lightweight (I had 12lb wheels on mine), but the Crossfire stops beautifully, corners with much more overall grip than the Miata and will outrun it in a straight line by a healthy margin. Plus it's quiter, stiffer and more comfortable all around. Yes, it lacks some of the Miata's overall tossability (you can't defy the laws of physics and something as light as a Miata will change direction like a housefly) but I've had the Crossfire on a road course and it was just a dream. Absolutely wonderful.

Looking at Road & Track Magazine test data panels for the current NC Miata and the Crossfire we see things like this:

0-60: Miata 7.0 seconds, Crossfire 6.4
1/4 mile: Miata 15.4 @ 88.7 mph (the '99 Miata was a little slower than this), Crossfire 14.7 @ 95.9 mph
Skidpad: Miata 0.86g, Crossfire 0.89g (this is actually a very noticeable difference)
700 ft Slalom: Miata 67.7 mph, Crossfire 67.5 mph
Braking 60-0: Miata 112 feet, Crossfire 113 feet
Braking 80-0: Miata 200 feet, Crossfire 203 feet

I used these numbers because they were done by the same magazine using the same test methods and even the ambient weather conditions were similar for both (+/- 1 degree F and 1% humidity). But these are just raw numbers and say nothing about the actual driving experience. I loved my '99 Miata but the Crossfire just offers so much more of what I want (I know lots of guys at miata.net will argue this!!) and I bought a "new" '05 Limited Roadster for less money than a new '07 Miata would have cost me.

But back to the orginal question about wheel size/weight and performance. Sure, putting lighter wheels on a Crossfire would have some benefit just as reduced unsprung weight on any vehicle would. But the more a vehicle weighs and the more power it produces, the less important a few pounds of weight makes. The situation with the Miata, especially the first two generations, is that they so light and so low on HP and torque, that larger wheels have a huge negative impact on all performance parameters. It's no wonder then that Miata owners get slammed at miata.net when they want to put 25 lb, 19" wheels on their little cars. Those wheels would transform a delightful little sports car into a ponderous, sluggish economy ride.