It entirely depends on your crank and cassette selection. Since you're running an XT group, you have a very wide range of cassettes to choose from. Your crank, if a double, will likely be the 40/28t model, vs. a triple which is 42/32/24t. I wouldn't worry much about losing your top end. Loaded, you're not going to be winding it out much on the flats, and downhill you're likely coasting anyway. Now, for the low end, you're losing a bit since your low gear will be a 28t instead of a 24t. However, depending on your cassette selection, this can be all but voided. If you run a 12-32 on the triple, you'll get about 95% of the same gear ratios by running a 11-34 with the double, for example.