I went from a 280x to a gtx 970 - mainly for power /temp reasons as I've turned my Silverstone htpc rig into a gaming machine now.
The 280x was reaching 80c+ in my cramped setup which I didn't like.
Performance wise the boost isn't massive (maybe 10fps or so) but it can & will ultra any game without worrying about graphics settings at all.
The benefits mainly are the power draw is around 50w less (should be more but my card is the Galax one with 8/6 pin connectors & 8 power phases & I've upped the voltage limits to the max in afterburner) + in game it sits around 62c which keeps my whole system cooler & quieter which was my main purpose.