i edit h.264 on a 3 years old machine in fcpx just fine without needing to make proxies. (3,5 ghz i7, 32 gig ram) - as long as i don't want to playback more than two streams simultaneously. encoding everything to proxy media would need more time and harddrive space than i'd want to spend. but i guess, when 4k, h.265 recording becomes mainstream in a few years (afaik only samsung uses it right now - and they got no professional video products anyway), i'll have a faster machine.
regardless, i think we will see h.265 support in imovie/fcpx in fall, early next year at the latest.