XF Downunder, You didn't say if your car was a automatic or a 6 speed. If it's an auto, then changing the rear end to what ever ratio you want would be OK. But if it's a 6 spd. going to a lower gear ratio much more than say a 3.55:1 would render 1st. gear practically useless.
This idea has been discussed at length many times hear. HDDP has done alot of research about it because he races his car and as of yet, hasn't come up with any affordable way of doing it. From what I can tell, since our car has SLK "mechanicals" that gives us very few options.
I've mentioned numerous times to at least try the front tires on the rear to see if that would make any difference, because of there smaller diameter, it would essentially give you a ratio of approx. 3.50:1. You would simply need to borrow somebody else's front tires though, because I don't think the rear tires will clear the front fenders.
Good Luck trying to beat your wife's HEMI, they can break into the 13's, and I think it would take at least a 3.91 gear to get a Limited XF there.