also how much of an increase is broadwell over haswell or is it just a more efficient variant
Yh...mate it won't be a big difference 10% or less mate. maybe efficient but I don't think it worth waiting for broadwell like seriously, if the CPU is not 30% better which is highly unlikey there is no point waiting for broadwell. The broadwell series will be 10% or less difference with Haswell series, including the "E" series there won't be a difference worth waiting for mate.
Source is from looking at the pervious generation, like haswell and ivybridge there isnt even gap worth getting excited or admiring about. The sandybridge is not even that far from haswell and that is 2 gens ago