Before I tell you that you'll be just fine, I need to confirm a little information about your configuration.
The PCIe x16 slot on top shares bandwidth with the second full length PCIe x16 slot (runs at x8). My question for you is: Do you have any expansion card installed in the second full length PCIe x16 slot?
If there is one installed, then the top slot for the GPU will be forced to run at x8, reducing bandwidth and slowing the GTX950 to just below full performance.
If there is no card installed in said slot, then you'll have no issues whatsoever.