personally I wouldn't worry about the performance hit, even at 5/6% at the most you can always overlclock the GPU's and reduce that percentage/gap. Its not like your games are going to lag, 8x is plenty unless you absolutely need to get the highest benchmark score you can possibly get.
The issue that happens from having 16 lanes to 8 is technical and complex.
There is two way communication, through the pci-e controller, cpu, gpu. They made the electrical specification larger to stop 'potential' bottlenecks. Overclocking the gpu, speeds up gpu performance regardless.