This is always a compromise, everything is with wind. Transmission efficiencies are high at full load to the point of being negligible. Near cut in the loss will be much greater, a chain drive is best, timing belt is reasonable and a "v" belt will be poor, gears are big problems for wind and best avoided.
Yes the extra bearings will add a bit of loss and so will the slip rings but if we are looking at an air gap alternator I think this arrangement will do better in the region near cut in. If there is useful power there then fine, a few % of near nothing is not likely to be worth chasing.
If this is an iron cored alternator the core loss will increase with speed so the gain is less so the chain drive may compete, belt probably not.
The low speed alternator will be way more expensive for the same power rating and efficiency so you have to decide where the trade off is. The implication seems to be that the gain is only in light winds so the high wind efficiency may not matter for a very low wins site.
I am not convinced that wind energy below 6mph is worth chasing but that is not my concern. For realistic winds even with a HAWT a chain transmission is well worth considering for many conditions, its main snag is complexity rather than efficiency in real wind areas.
Flux