You should ask heat-troller_mike.
I thought the way the controller works is that when it's set at, for example, 50%, it would cycle the gear on and off several times a second, so that the total On time is 50% of the time. If so, then that means that when it is On, it's drawing the full load of power (i.e. 90W) - but only very briefly. That seems like it means the bike would need to be able to handle a 90W load, whether you ever run the jacket at 100% or not.
That said, I personally am all about future-proofing. So I got the WnS jacket liner and the dual remote controller. Then I ended up adding the WnS heated socks and I am glad I got the dual controller! I turn on the socks before I ever turn on the jacket and generally run them at a higher setting when they are both on.
ps. In case you didn't know, Warm n Safe makes the heated gear that First Gear sells. It's identical, just with the FG label on it.
Iron Butt # 4999, (former) WERA/CCS EX # 666, AMA Life Member, Drummer, Bassist