Reg tow vs MaxTow will make about a 400lb difference. Going from an XLT to an FX4 is probably not much in trim difference. It's the tow-payload packages that really make a difference. The MaxTow has a GVW around 7650-7700lbs vs the reg tow at 7350lbs or so.
Certainly an XL trimmed 7350 GVW will have a higher payload than an FX4 7350. But trims closer to each other don't vary all that much. I was lucky enough to find a 2010 XLT MaxTow a few years ago that had an 1857lb payload. Still wasn't enough for my 950lb TW, truck cap, firewood, two bikes, two 7 gal water jugs, lawn chairs, Weber Q, etc. You get the point. I was maxed on the RAWR at 4000lbs. Truck had a RAWR of 4050lbs. Thus the 3/4 ton I now own.