Bear in mind that for the last couple of years touring cars have been running 5 NiMH cells in modified class, just 6V nominal voltage. Performance has not been a problem, drivers are running a slightly hotter motor wind than they would have done with 6 cells, the cars (albeit running at a lighter weight limit) are faster than they were despite less voltage.
If you are in a modified class, 6.6V in itself won't be a problem, just drop a wind.
In spec classes the 6.6V pack will never compete unless an "equivalency" is introduced - although equivalency rules never work, one setup always has a benefit depending on the track.
Unfortunately I don't know enough about the LiFePO4 cells to comment on how they can handle high current draw. I know that they are more robust than LiPo cells, which is why Tamiya have made a pack of them which is a direct fit in a normal car - but the capacity of that cell is only 2200mAh, which doesn't compare very well with the 5000mAh+ available from LiPo at present.
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