I can't see that the extra weight could account for the OP's observation. Yes it's heavy, but it still only amounts to about 3 or 4 extra pounds or about as much as a couple extra full water bottles. Sure that'll slow me down a bit on a climb, but it won't make as drastic a difference as reported. AFAICT, NuVinci has been very secretive about efficiency estimates and has given out no information about it. And I have yet to see a review with any quantitative data. But the reviews I've seen have not reported such large efficiency losses as the OP seems to have experienced.

Please let us know if you resolve the efficiency issue.