Pci-e 2.0 isn't saturated yet, and most definitely not by a 7950.
Your i5 isn't a bottleneck to a 7950.
You're trying to micromanage something that can't be micromanaged. Different programs/games will use the CPU/GPU at different amounts and also differently at specific areas in the game. Worry less, play games more.
+1 Pci-e 2.0 is still fine for current GPUs. And an i5 is not bottle necking your GPU, it's all about how the game was programmed, different games use CPU cores more efficiently. Enjoy your rig, it is great for 1080P, HAVE FUN!