Well, if you want longer range, having greater power flow capability is actually detrimental, because you use up more power on every startup than necessary, unless you really need quick takeoffs.
For longer range, you want to lessen those peak amps wherever possible.
Wh/mile is just that--how many Wh you use in each mile. So to calculate it, do exactly what the Wh/mile name suggests--divide total Wh by total miles. If you do this for any data collected on trips with other chemistry types and/or motor/controller setups vs the current chemistry and setup, you can see how "efficient" each one might be.
The lower the number, the longer your range.
Total usable Wh in a pack, vs Wh/mile usage, determines your range. So if you have a pack and setup that uses twice the Wh/mile that another setup does, but the pack isn't twice the Wh capacity of the other setup, it won't even get the *same* range, much less more, even though the pack is bigger.
Brush problems can contribute to motor heating and inefficiency, but brushes usually last many years. Brush holders should, too, in a well-made motor, but lots of them are pretty cheap plastic or whatnot, that deforms under heat and prevents proper brush seating. Same with brush springs--they can lose their springiness if they get hot enough, and no longer hproperly push the bursh against the comm segments. Brushes do wear away, and need replacing, and that you can check visually but you have to open the motor up in most cases--unless something is definitely wrong at the motor there's no reason to do that.
If the motor does not get any hotter than it used to, and doesn't really get all that hot, it's probably not the brushes.
If it were windings, it would probably not work at all, or be fairly obvious something was wrong with it, by noises, or cogging or smells or whatnot, compared to how it worked just before that failure. Not usually a gradual thing; you'd probably know if it had a winding problem right away.
1200W vs 800W--a motor is "rated" for a specific max continuous power. If you exceed that, it may get hotter than it is supposed to, and that could damage things inside it. Sometimes the damage is instant, more often it is gradual, over a long time, as things heat and cool and heat and cool, and sometimes nothing at all happens because the mtoro is very conservatively rated. Other times it's rated right at the edge of what it is capable of, and running it higher could kill it the first time you do that. No way to know without testing, but you can expect that most cheap motors are probably not that conservatively-rated.
For the rest of it you will prbably want to poke aroudn the ES wiki, and do some searching and reading on the ES forum in various threads, especiallyin the technical reference section.
