CPUBoss comparison - agrees with the majority of sites that I've seen, but some sites offer opposing results/opinions.
You won't see much difference in gaming from swapping to a Z97 platform. Some games a tiny bit better, some a tiny bit worse. You would see much larger differences if swapping your GPU card. Honestly, the differences between your top-end AMD system and a top-end Intel system wouldn't be enough (in my opinion) to justify spending on upgrade ... I would wait until 2016 for Skylake and new X99 processors and such, it would be a lot more bang for your buck and a significant linear upgrade instead of a somewhat costly marginal nonlinear upgrade.

990FX+SB950 is slightly inferior to Z97, but this is a non-issue on high-end motherboards (with either chipset) because the manufacturers tend to integrate various add-on chips to round out hardware compatibility. Asus Maximus VII Impact and Sabertooth Mark S (Z97) motherboards support essentially the same add-on features as your Asus CVFZ (990FX) motherboard.

The overwhelming majority of GPU cards (including your Asus-overclocked R9-290X) can't fully saturate PCIe 2.0 bandwidth. But if you're looking at something like a pair of 980Ti/TitanX cards then Z97 would be a better choice because the lack of PCIe 3.0 on a 990FX might throttle maximum fps. And of course, all AMD motherboards support CrossFire but few AMD motherboards support SLI.

Z97 mobos tend to support faster DDR3. But extreme DDR3 speeds are rarely realized when overclocking either the FX-9590 or the i7-4790K, since the processors dictate upper limits on memory performance more than the motherboards. Your CVFZ supports up to 4x8GB DDR3-2400 ... but if you wanted the advantages of extreme memory (up to and beyond 8x8GB DDR3-3300) supported by some of the top-end Z97 motherboards then you would have to add it into the upgrade price.