I don't use Always On, since my Series 4 doesn't support it. But if I had a newer model and if I was having battery life issues and was using it... yeah, I'd shut it off to see if it helped. If it did help, I would just leave it off. I don't care whether people see a dark screen on my wrist.
As to reducing screen brightness... that's a classic for any battery-operated device with a screen. It can be very effective.
But I seriously doubt, unless you currently run screen brightness near 100% and spend a lot of time with the display on, that you would actually get 2x battery life from these changes, or that Always On would turn out to be a major part of that improvement. Always On was designed to be very battery-efficient - lower brightness than "normal," with only a limited part of the display illuminated. Overall brightness seems likely to be the bigger contributor.
You could test that, of course - once you have measured your actual battery-saving gains by comparing your battery consumption before making the changes to after making both changes, then enable Always On and make more measurements.
To do this sort of thing right you need to make a record of the time and battery level when you first take the watch off the charger, then the time and battery level when you put the watch back on the charger - and keep recording that over a series of weeks to get a good, long-term average for each variation in your settings.
Best thing, of course, would be to have an app that stores long-term battery/charging data and lets you export the data for analysis. The apps I've looked at generally show a 24-72 hour record of some of these things, but if they allow long-term data collection/trend analysis, I haven't seen mention of it in the app descriptions.
I'm kind of wondering what benefit I'd get personally from 2X battery life. As it is, I wear my Watch as much as 22 hours between charges - sometimes I remember to put it on charger before I fall asleep, sometimes I don't. If I don't, I put it on charger for an hour or two after I wake up. I think I've received a low-battery warning no more than 10 times in the past year - usually in the morning after realizing I'd failed to connect to charger, or the charger accidentally disconnected in the night (perhaps waking in the night, realizing I was still wearing my glasses, placing them on the night table and inadvertently bumping the watch off the charger).
Personally, I'd much rather have a daily charging routine (charge it every night/morning) than a two-day charging routine - it's a much easier habit to maintain. Sure, I could create a Reminder to help keep me on schedule, but it's far more likely that I'd just stick with my established habit. It seems a bit silly to create two reminders: "Charge your Watch tonight" and "Don't charge your Watch tonight."
