I doubt Broadwell will have any performance improvements over Haswell at the same price points. You're probably better off buying a cheaper Haswell then if you want a cheap Steambox. Intel has pretty much given up on improving overall performance of its chips. IVB was only 10 percent improvement over SNB, and Haswell only 5 percent over IVB, and the difference in price between new-gen and last-gen is probably more like 30 percent.

Intel’s Ben Widawsky, who works on Intel’s Linux graphics driver efforts, says that “Broadwell graphics bring some of the biggest changes we’ve seen on the execution and memory management side of the GPU… [the changes] dwarf any other silicon iteration during my tenure, and certainly can compete with the likes of the gen3->gen4 changes.”

This combined with the fully opensource Linux driver for Broadwell would mean that there is a very high chance it would perform significantly better.

Broadwell will perform better, but Intel may decide to charge more for it. That's what they did with Ivy Bridge and Haswell.

Single core performance in CPU intensive applications really improved between Nehalem/SNB/IVB/Haswell actually. [1] is a benchmark of the Dolphin Emulator (mostly single threaded performance) on a variety of recent CPUs that shows this.

[1] https://docs.google.com/spreadsheet/ccc?key=0AunYlOAfGABxdFQ...