The cheapest thing you can do is port and polish your throttle body, easy and free if you have some tools! That combined with an intake or DIY intake might help off set the tires... When i had my 08 WK i had an intake and an airram powerwire(simple IAT mod) that helped it out, got me mpgs and some extra power! As for the transmission it should be the least of your worries, its practically the same thing they use in the 300C with hemis and the srt8s..... WA580 is a Mercedes designed transmission, wish mine had it to be honest.....