About 30-40% on average, depending on the game (comparing i5-4690k @ 4,5GHz with FX-8350 at 5GHz)
But those are overclocked and like i said, i going to buy r9 280... so i dont think im going to get any boost with i5
He's right if you're talking raw framerates just running on processor power and excluding GPU, but the fact the i5 has an integrated GPU contributes a long way to that.
As part of a system, most games show a 1-5 FPS difference between the i5 and FX-8350. The i5 is technically faster, but not £100 faster. And it's between marginally and substantially slower on heavily multi-threaded and scalable workloads, like running multiple VMs, rendering, Cinebench and the like.
For most builds, in my opinion you're better off adding £100 to the GPU fund or buying an SSD as a boot drive than you are spending £100 more on an i5 build over an FX.