A 2003 SLK came with 205/55R16 front and 225/50/ZR16's in the rear.
In a comparison Road Test of the SLK 320 vs an 04 Crossfire, here's what Motor Trend had to say about their respective handling.

"The SLK 320 arrives with 16" wheels shod with Michelin Pilot HX MXM tires. Although it can't compete with the Crossfire in slalom or skidpad performance, the SLK's ride is more isolated and just feels more sophisticated despite the same, small 94.5-inch wheelbase. There's less tire slap and thump when encountering abrupt transitions and less tendency to porpoise. The Crossfire's tuning falls on the sporty side of the fence, while the SLK's is more luxurious.
Both cars share like-ratio recirculating-ball steering that feels sporty, but does not offer the light and agile feedback one might expect from such small cars. Also the two have identical brake components, but the Crossfire's wider and stickier tires help it stop shorter".

Installing this size tire will also make your car at least 3/10's faster in the 1/4 mile, will cost much less to replace, and will not have any effect on on your speedometer or odometer.