I had many of the same questions and did a fair amount of research. If you look at the Powerlet website, they have typical values for bikes with and without fuel injection and the consensus is that a fuel injected bike, like the Kawasaki 900, uses ~285 watts for the bike. With over 400 watts being generated (granted at cruising speed) there should be over 100 watts available for accessories (this also fits with the accessory fuse of 10 amps or 120 watts.)
I ended up getting Gerbing jacket liner and gloves and have not had any problem. I went for a one hour ride at 30 deg F with the jacket and gloves on at ~50%. When I got back, I plugged in the battery tender and once it went through the checks and started charging, it was fully charged withing a minute or so (this indicates that the battery was being charged by the bike's charging system.)
A couple of hints that I've seen in some of the forums are:
1) Use a Heat-troller of some sort and only use what heat you need. For instance, when the jacket is at 1/2 power, it is only drawing ~38 watts or so.
2) If you are using heated gear in city traffic, you may want to turn it off when idling at a red-light. The rated power output of the alternator system is at high rpm (I think it's rated at 5000 rpm.)
3) If it is real cold and you are using full heat, try to give the bike at least a few minutes of higher rpm driving with the heated gear turned off before you stop the bike. This will give the charging system a chance to charge the battery
Incidentally, you are better off using a solid state heat-troller from Warm-n-safe or Gerbing as opposed to a home-made rheostat. The controllers turn the heat on and off quickly as opposed to a rheostat that wastes the excess current through a resistor.
Good luck, you'll like the heat.