Im not exactly sure how you measured your couplers, but I just weighed and compared both sets of couplers a few days ago.
The yamaha OEM cast couplers AND dampener was 1 lbs 10 oz
The BILLET kawi conversion couplers AND dampener was 1 lbs 7 oz.
hardly half a pound. Perhaps yours are milled far lighter, but as for me, I think the widespread claim of everyone feeling a large snappy difference is a bit bogus. The yamaha coupler is roughly 3/4" larger in diameter, and though yes, there will be less centrifugal inertia, it wont be that much as compared to an aluminum flywheel when you are talking about loosing 3lbs on a 7" disk, not 3-4 oz. on a 3" disk. Personally, unless your grenading the yamaha couplers I think its a bit of a waste of $100+ . (i also dont understand the recent hype with the billet timing covers either... but thats another story)
On the bright side it does look hot as




. =] Good build!