On the performance of what though? A gpu intensive game of course will see bigger improvements with a better gpu. Try playing a cpu intensive game like starcraft 2 with 8 players or supreme commander. A gpu wont make any difference to calculations required when theres a lot going on.
A gpu wont do much for reducing encoding time either or general application use
Dude I play games like SC2,SINs and SupCom and SupCom2 - if you actually played some of these games at LANs,unless ALL your mates have 10GHZ Core i7 6700K CPUs,games like SupCom are limited by the slowest CPU in the network.
Plus one of the BEST players I knew in SC2 was a decent Diamond League player and he was running the game on a Phenom II X4 965 and thrashing people having SB and IB Core i5 and Core i7 CPUs.
Also,some of the best players worldwide are running the games on laptops(not even the full sized ones),which most likely during extended games are not even Turboing fully due to thermal issues.
People who play these games competitively drop all the details down anyway - that is what I have seen.
Plus piddly little 20% to 25% increases mean bugger all if during the worst aspects of these RTS games - because 12.5 FPS is so much better than 10FPS.
Even with other CPU limited games like D3 during the Rifts,I do get better framerates than my mates with AMD chips with an IB Core i7 yet when the massive dips happen it happens to everybody and looking at the D3 thread people even with Haswell chips have seen the same.
Sometimes the limitations of online games are on the server side - EVE is a famous example.
Someone running SC2 or SINs on an HD6450 with a Core i7 6700K is probably going to have a much worse experience running it with an GTX970 and a Core i7 3770K or a Core i7 4770K.
Let's look at something like Total War:Attila.
The difference from going to a Core i5 2500K to a Core i7 4770K is much less than going from a GTX780 to a GTX980.
If you had a weaker card like a GTX680,it is even a bigger increase.
Its a total misnomer that people think many strategy games are not GPU heavy and are exclusively CPU heavy- more and more are using the compute capabilities of the graphics card to help the game run better or to increase complexity.
Games like Civ5,Total War:Attila and it appears Ashes of the Singularity are very GPU heavy strategy games as well.
This is why it makes far more sense if you have a reasonably CPU already to upgrade the card first if its not that new,for most games.
Compare the increases in graphics cards performance from generation to generation.
GTX680 to GTX780TI to GTX980TI.
A GTX980TI is twice as fast as a GTX680,and that happened in three years.
CPUs did not get anywhere near that especially for those overclocking
over the last FIVE years.