for Consistency I think we should really stick to IPC "above" not below. "i.e Broadwell E will be xx% higher IPC" and yes it makes a difference. 20% lower, means 25% higher
I'd put my bets on a 10-15% advantage for Broadwell E.. on average - mixed workloads. but it's going vary enough that anyone could be 'right'. The arguments that will ensue as people try to defend their predictions will be pretty painful to watch. there'll be cherry picked results from all corners. . Without a 'standard' set of benchmarks to draw an average from, it's not going to be easy.
but alas, if we're talking an avg over a large, reasonably varied benchmark suite, anything less than that falls outside AMD's claims for one, and would also be a bit disapointing given what we know about the core.