You can tell if your CPU is holding you back by lowering graphical settings. If your framerates don't rise, your CPU is what's limiting you to 15fps. If they do rise, you're running graphical settings too high for your video card.
Maybe? Depends on what you use it for. At stock speeds the 6600K is close to twice as fast as what you have, but once your CPU is at a certain level (for your specific tasks), more speed doesn't help.
The i5 will be noticeably faster in most recent AAA games, such as Battlefield, Mirror's Edge, thart sort of thing.
You can tell if your CPU is holding you back by lowering graphical settings. If your framerates don't rise, your CPU is what's limiting you to 15fps. If they do rise, you're running graphical settings too high for your video card.