For example GTX970 works well when it is inited with GPU (geo+tex) cache less than 2GB. Then restart IPR closing and openning the IPR window again. And then while IPR is running you can up you caches more than 2GB: go to Hardware Settings, up the caches like you want and click "Apply". And it will have both good speed and larger caches which may be better for large scenes.
Sometimes GUI lags are possible when you render on the GPU which is also attached to your display. This happens because one GPU is shared among OS programs, Cinema viewport and CentiLeo.
But this trick may help eliminate these lags and run smooth even if you have one only GPU.
This thing has relation to CentiLeo's out-of-core feature and NVIDIA CUDA drivers which also compete for memory.
When we will work on multi-GPU feature we are going to make this process automatic so that you will have no headache with such tricks irrelevant to creativity. This is our aim!
PS: One more recommendation is to render on one or more GPUs that are not attached to display. And leave "display" GPU to the OS and viewport, it can be just some basic small GPU.