From what I've been told, by slk320 owners, it's worth it if you can afford it! The car will have the power of the SRT6 with a 6spd. Too much fun to be legal. After that get the posi-unit upgrade to lay the power to the ground evenly, especially for going around corners safely.
edit: being that you can now get a low mileage Crossfire Base model for $16,000, and the Kleenman kit is another $10,000, your gonna be pretty hard up to find an SRT6 nowadays for $26,000. They are getting harder to find and the dealers are getting wise to what this car really is, and have been pushing the price upwards on SRT back up to $30-$34,000 lately. Not to mention the suspension is known to be a bit harsh on rough roads. but you are getting $2000 worth of brakes and stiffer sway bars.