Just to level set:
802.11ac is still somewhat off in the future (1-2 years before it really picks up steam...remember the long pole in the tent will be when do the clients start supporting 802.11ac en masse. iPad 9 ? iPhone 8 ? ;-) ).
Of the vendors that have discussed 802.11ac over the past few months, I don't recall seeing any 'requiring' new hardware on the controller side of the equation. Instead they mention (and you can count on) new AP hardware being required ( a function of new chipsets being needed for the APs to 'speak' 802.11ac).
If you rewind the clock for another datapoint, you will find that when the 802.11n standard came out (circa 2007-2009...yes it took years to get that last bit to the finish line) we did not 'require' new controllers to support that standard. Instead a new family of APs (the 1xx series) was produced in 2007 response to 802.11n and could be used with any of the controllers made well before the standard was even conceived.
Will faster, more powerful, futuristic controllers come out from Aruba? Definately... technology always evolves of course.
But ponder how long will it be into the future will it be before a network becomes saturated with 802.11ac clients, such that they would require(aka. benefit from) more processing + more performance than the existing controller fleet can provide?
Conservatively? Years... makes investment in today's technology (802.11n) and 3000/6000 controllers a sound decision while enabling a 'sprinkling in' of 802.11ac when it makes sense 18-24 months from now an onwards.