With respect, +50 watts gives you a 25% improvement in alternator capacity, vs. 2%.
I certainly agree with you that many stock regulator/rectifiers on Japanese bikes are still inexplicably wimpy/poorly provisioned, so upgrades there certainly make sense. I had always expected back in the day that they'd get better, but so far...
LED off road lights sound like an excellent idea--no street lights where you're going. Adding HID headlights would drop your highbeams (you won't be using the lows off road) from 60 to 35 watts draw. If the bikes don't have LED tail lights, that might help drop power draw as well.
Ian points out it'll be August, which means mostly warmer temps, but in the mountains and in the dessert it can get quite cool at night even in summer--and these bikes are unfaired. A vest and/or grips might make quite a difference if you allowed the capacity for them.
Plus, these bikes aren't just for the trip, right? So even if you don't need the electrics on the TAT, you might well want them in "real life" riding in the future
Just sayin'
Since these bikes have racked up literally 10s of millions of miles over the years in every climate, condition, and terrain, stuff like gaining small % of wattage is way down the list for me in terms of mods (although have always been up for replacing regulators/rectifiers with better units). Of course it helps that I've become better at keeping my "inner engineer voice" quieted down; to avoid focusing on the coolest, most electrically focused, or most difficult 2% gain items
LoL. It never even occurred to me to think about power clothes for this bike, I can always pack a 0-volt sweater :-)....the only electrical things I'm considering are LED off road lights (small power draw+BIG lumens) because I actually like to ride at night, GPS/iPad/Phone power, and that's about it right now.
Top mods: Seat, Luggage frame (haven't decided on hard panniers or soft yet), bash plates, windscreen