Higher clocks more shaders but they sacrificed on other aspects, the MSAA issue is not limited to the Bus either but a combination of everything.
Obviously yes power and heat but you said "one stronger better than two weaker" which is not the case here particularly with a 670 not being the top card either. Perhaps 7850's but even then those cards will almost use the same power as a single 670 anyways.
I personally don't see the bad in 2 660 ti in sli.The bandwidth in each of them may be relatively low but it doubles when used in sli providing 288.4 gb/s which is enough.What doesn't add up is the vram but once again 2gb is enough unless you opt to use triple monitor displays.
To simply conclude, 2 660 ti in sli is the most bang for the buck for the end users.
I'm using a single 660 ti for my full hd system and I don't find kepler architecture flawed.
i would have gotten the twin frozr 4 from msi but it wasnt on sale
I'm using one and its simply amazing.
The fan speed is kept to 34% by default and at maximum load rises to 44%,still is enough to keep the gpu cool and its uber quiet.
I have tried pushing the fan speed to 100% in one bump and it sounded like a jet engine starting.I so... love this thermal solution.