If your bike currently has a flat bar and you are willing to run all of the pieces from the XT group, you shouldn't have a problem with it all working together. But if you want to run STI or even barend shifters on drop handlebar, the XT stuff doesn't work with any road components.

As for the top gear, this gear calculator lets you compare gears from two different systems. A 36/11 top gear spins out below 25 mph at 90 rpm. A 50/11 top gear spins out at 33 mph at the same cadence. That may not seem like much but from my experience, a bike that spins out that low can be frustrating. It doesn't take much of a downhill to reach 25 mph and that can be a lot of coasting.

Now compare a triple 48/36/22 to your current set up. You have almost the same high and have a much better low than you currently have and a wider range of gears. You'd have to change your shifter to one that is capable of shifting a triple and you'd probably have to change the front derailer. Chain Reaction has a Deore Trekking crank for about $75 US (get the 48/36/26 and a 22 tooth inner ring). This route would be cheaper than a whole group and would give you a better gear setup.

All of Shimano's Hollowtech external bottom bracket cranks are designed around a 73mm bottom bracket. You just use spacers to move take up the space.

This isn't a problem either. Again, you use spacers to move the crank to the position that it needs to be. Spacers come in a variety of widths so while it's not infinitely adjustable, it's pretty close.