I think it will be hard to tell. Obviously a beefier chip requires more bandwidth to keep it fed, but once you're past that bottleneck, throwing more at it won't add more performance. This is the reason why OCing a 290X's memory doesn't do much either. Besides a smaller package and lower power consumption, it will also be hard to say what other gains HBM brings vs GDDR5 since we don't have two versions to compare. Like let's say Fiji with HBM and a Fiji with GDDR5 clocked high enough to have the same bandwidth. This probably sounds nitpicky, but I assume you get my point.
And yeah, I've seen that ~1GHz figure being thrown around too. I'd say Fiji will probably be about 40 to 50% faster than a 290X. Anything higher than that would be a nice surprise though.