Which older drivers? You clearly didn't read their steam forums, in early 2014, AMD users have been telling the devs of broken performance.
Check the thread:
http://steamcommunity.com/app/234630/discussions/0/540734792953723817/
Devs thinks its just bad AMD drivers, because AMD performs worse even in GE games... look at what he wrote:
"We do not favor anyone, we work closely with both. And there have been performance improvements lately for both sides, we even got to the point were we had to remove an optimization because it wouldnt work with AMD cards, "penalizing" nvidia users.
Go read a bit more about AMD drivers optimization and how they work, consult info on what are the similarities between developing on PC and Next-Gen consoles, you'll see it's nothing like you imagine and much different."
A gamer responds:
"If your theory is right then every game relesed for PC would run like crap do to poor AMD driver optimisation and that is not the case, the problem here is that sms is playing favorites and people like you with a democrat like approach to the issue doesnt help."
Dev response:
"It's not a theory it's a fact, check Tomb Raider, Battlefield 4 (non-Mantle), Bioshock Infinity FCAT results and see for yourself. All these games are AMD Evolved (Optimized for AMD) and even so they get poorer results. So please don't come here and accuse us of being biased."
Clearly the developers don't think much of AMD at all, didn't even bother to seek AMD's assistance (didn't inform the backers of any action regarding cooperation with AMD, except only that its their drivers, which happen to work fine in other games as pointed out my users).