I have a lot of experience with heated gear and riding year round through New England winters, crossing the Rockies multiple times, and I came in 2nd place one year for the ST-Owners Coldest Ride contest with a score of -12F. (I cant find the -12F photo, but heres a 3F one.)
Most FJR members have no problem running heated clothing, but there are a lot of variables that come into play. Are you running just a vest, or are you running a pair of riders with heated jackets, gloves, pants, and socks? As for the power draw, I'd recommend reading this forum they explain power better than I can. thread:https://www.fjrowners.com/forums/9-fjr-technical/120906-need-more-battery.html
As for the clothing I like my Gerbing heated jacket and gloves. I'm told a vest can be almost as good as a jacket because your body will circulate the warmed blood to your limbs. I have 2 complaints for my setup. The wires for my gloves plug into the cuffs of my jacket and if aligned poorly when my suit is zipped tight the wire presses into my wrist tendons and makes me sore. My other complaint is if I wear a fleese neck gator to keep the wind out the heated neck on my jacket sometimes overheats.
I really want to have heated insoles in my boots, but routing the wire would be a hassle so I occasionally move my boots over the pipes to warm them up on long streatches, and pack the disposable toe warmers for when i really need them.
I've never felt the need for heated pants.
TLDR: Start with a jacket or vest. Expand if necessary. Wearing a heated jacket is safer than 4 insulating layers because it restricts you less, so I think of it as safety gear.
Feel free to ask followup questions, I tried to not be too wordy and go off topic.