You will be fine. I know ppl who use 400w MH
's on tanks that deep.
Those XM 10k's are AWESOME! I thought the 175w Iwasaki's were bright.....these blow them away. They are SUPER BRIGHT WHITE. I was a little worried I would light stress the corals so I got some eggcrate and 2 layers of screen to cut the light penetration, but it seemed alot darker than before with the 175's. I got in touch with Eric Borneman again and he told me to remove the screen and just use the eggcrate since that will diffuse the light enough not to cook them yet still have an increase in PAR. I'll leave the eggcrate in place about a week.
Did I mention I love this 250w XM 10K + VHO