1. I'm sure you've read the following.

"Our tests demonstrate fairly little difference between a $225 LGA 1155 Core i5-2500K and a $1000 LGA 2011 Core i7-3960X, even when three-way graphics card configurations are involved. It turns out that memory bandwidth and PCIe throughput don't hold back the performance of existing Sandy Bridge-based machines. "
http://www.tomshardware.com/reviews/gaming-cpu-review-overclock,3106-4.html

2. There are many reasons for FPS drops from high counts of objects on screen to other. THIS IS NORMAL. Only your newest folk think that an uneven frame rate is a sign of a problem. Let's read Tom's on that. Example:
http://www.tomshardware.com/reviews/graphics-card-myths,3694-3.html

There's so much more about.

3. >> MOST IMPORTANT << since you the the 290. Read this now.
http://www.tomshardware.com/reviews/radeon-r9-290x-thermal-paste-efficiency,3678.html

Now that you know item 3, you know if the case temp rises it can cause the GPU to throttle sooner. Some folk dig in their heels on doing the work on the GPU or leaving the case cover open. I can't guess why.
Bob