According to what I've read so far,everything in the XF's drive train is supposed to be the same as the SLK's, ratios and all. I'm not sure, but I'm guessing that the diameter of the XF's 255/35-19 is the same, or close to the SLK's 225/50-16s. After all you only need 1.5 inches of side wall to make up the difference in wheel size, and we're talking 50 series vs 35 series.
Motor Trend magazine's Dec. 03 issue had a comparison test of the SLK vs the XF, both were automatics. Even though the SLK weighed 70 lbs. more, it still beat the XF in the 1/4 mile by .24 sec., on tires "skinnier" than what the XF uses on the front. This is the reason I suggested using the front tires on the rear to lower your 1/4 mile times. I think it would perform more like you were running 3.50 gears instead of 3.27s. The best times I've read on the non-supercharged stick shift XF is 14.7. It wouldn't surprise me if it knocked 2 or 3 tenths off that. (Assuming you get a good launch without to much wheel spin). Just practice.
I have to commend you for looking for inexpensive ways to improve your cars performance without throwing tons of cash at it, and keeping it mostly stock. There's nothing like spanking some "Tuner Boy" (at your local drag strip) who just spent his whole paycheck on a new "Wright Brothers" wing and a "fart" can muffler only to have him ask, "what did you do to your car?"
And you can reply,"nothing, it's stock".