With the 4" SL.... i think it may be a good setup.

Not 100% sure though, i'd definitely have to take a closer look at what wheels/tires some of the other 4" lifted XK's are running to feel a bit more confident. The width is the only thing that has me a bit "iffy".

275/60/20 would be another tire size to look at. They're 1" taller than the 305/50/20s, but a good inch thinner. (i'm a fan of the 305s though in terms of looks)

Edit: or 275/55/20.... 275/60 was popular in the GM truck world, but 275/55/20 would probably be a better option for our XK's.