For example a Radeon 6990 has 5.2 gigaFLOPS of computing power and yields roughly 800 megahash/s in bitcoin mining.
Apples to oranges. If all Bitcoin miners everywhere used Radeon 6990s, then you could say that the mining collective could
compute at (big number)FLOP/s if they directed their computational capacity at floating-point problems, but right now we're computing at 0 FLOP/s, simply because it's all integer based. You can't exchange floating-point and integer operations like that.
And the Tianhe-1 is computing at 0 khash/s because it only does floating point operations. That doesn't mean it can't be utilized to mine at 1,433,600 mhash/s. (7168 Nvidia Tesla S1070 GPUs at 200 mhash/s each)
Of course it's theoretical, why would anyone assume otherwise