I think it's more than 15-20%. I've upgraded numerous 920 and 870 systems to Ivy Bridge and in CPU limited applications the difference is quite immense. Of course, most folks just look at GPU limited gaming benchmarks and immediately brush such differences off, but I can state that most applications are not GPU limited. Further, most games are not GPU limited either - but most people like taking comfort in their 5 year old systems by staring at GPU limited benchmarks. Even if you're a gamer not every game is a Metro: Last Light - CPU makes a big difference in strategy games, MOBAs, MMOs, among a variety of other games - SC2: HOTS in particular benefits tremendously from higher CPU speed. Anyway, I feel like some folks wear blinders and see what they want to see when staring at benchmarks for their C2Q and Nehalem PCs...(By the way, this is not in any way directed at you, crashtech - just a general observation)
With all that said...OP: if you're upgrading now I would just get IVB-E. If you're upgrading early next year, Haswell-E makes more sense but be aware that the costs for Haswell-E will be generally be higher - this may or may not matter to you. DDR-4 and SATA-E will be new features with Haswell-E and I have absolutely no doubt that both of these technologies will be expensive at launch. On the other hand, all of the tech associated with IVB-E is well established and reasonable in price, IMO. You also get all of the current technology that you need such as SATA6G and PCIE 3.0. That (added cost) may be a consideration for you, if cost matters a lot to you.