Not familiar w/the head but sounds like your assumption is correct (2 8ohm cabs would get 250 watts each). Probably more w/peaks. Will you blow a single cab? TOTALLY depends on how hard you push it. I saw a guy play a single 1X12 cab w/ an 1100W amp (on tour). Sounded great. What are you volume needs is the pertinent question. Use your ears. If the cab is struggling add another. If not, you are fine.
Having said that, having 2 cabs (ideally matching) has it's advantages, as long as you have the cash and don't mind the schelp. They look bitchin', the extra height makes it easier to hear yourself if you are right in front of the rig, and you can leave one at the doghouse (rehearsal space) and one at home for practice. Just bring the head back and forth, and bring both cabs for gigs that may require the extra volume.