I used a couple of makes and did not see much benefit in top down.
Best furler I have used was a Karver btw. I also like Facnor units.
You must assure your sail can be made tight between the head
and the tack which also includes in this case the top swivel, the lower unit (drum) and some form of purchase
(unless you make the luff tight with the halyard
alone). Some genakers will not furl then unless you add some form of a prod to your boat.
With an older design genaker, you will likely have to modify the luff to allow for the non-torsion luff line.
All in all, if you do use the sail often and if the boat is big, go for it. Even the pre top down units make life much easier with big sails
On smaller boats the sail can be just lowered in the lee of the main. An easy task if the boat has a reliable AP.