As you may have heard in the news, NVIDIA has been falsely advertising the GTX 970 to have 4GB of memory on a 256Bit bus with a peak transfer rate of 224GB/s. It turns out that the actual transfer rate of the last 500MB of those 4GB is almost a magnitude lower. This can cause significant performance degradation when that memory is accessed.
Memory bandwidth is crucial for an application like Cycles to perform well, however since the data is accessed more or less randomly, the performance hit might be amortized.
It would be interesting if any owners of a GTX 970 could report on any performance degradation when using up those last 500MB in a Cycles scene. You can use a tool like GPU-Z to monitor your memory usage.
UPDATE (Juli 29th, 2016):
NVIDIA has agreed to pay 30$ per device in a settlement of a class action lawsuit. Those who have purchased a GTX970 may be entitled to this sum (after filling out some paperwork).