From comparing the speeds I'm getting to hdminer, the miner isn't 6+% faster, it's only 1.2-2% faster.  I'm using Catalyst 11.4 without downgrading and underclocking the memory so the difference could be even less than that (11.5 is supposed to give it a few Mh boost and not underclocking it would increase hashing rates by 0.5-1.5Mh/s).

Here are the rates (per core) I get at various speeds in the overclocked position using the phoenix 1.48 miner with the poclbm kernel:

880 - 379.8
900 - 387.0
905 - 389.6
910 - 391.9
915 - 393.4
920 - 396.0
925 - 398.2
950 - 408.5
955 - 410.8
961 - 412.6
966 - 414.6

Those numbers are near the higher end of the spectrum but not the very top.  The numbers are sustainable and not just one time blips that timing/thread switching might inflate.  At 880, hdminer was actually 1.8% slower (759 vs 746).  For the 915 and 960 settings, hdminer was faster (@915, 786 vs 802 = ~2.0%, @960, 824 vs 840 = ~1.94%).  These are using numbers that would favor hdminer more (lower hashing rates than optimal and rounding my rates down).  More optimal numbers would probably put it closer to a 1.2% difference.

Using $17 as the value of a BTC, 250 BTC = $4,250.  If the cost of a 6990 were $740, that'd be 5.74 6990s.  Assuming hdminer was 2.0% faster, you'd need over 295.6 6990s to make a profit.  So unless you're running an operation that exceeds 248GH/s, you shouldn't buy hdminer.  I'd be surprised if there are several people with that much capacity, let alone even one.  It'd be even worse if hdminer were only 1.2% faster.