as expected, a lot of different and valid views on that
what got me thinking about this first time was actually decades ago in the original game. i had a guy that was that well trained, he had actually enough health to survive a direct blaster shot, not kidding (was like a year in the hospital after that ^^).
enough TUs to run about anywhere on the map, so completely OP and ridiculously superhuman. but no matter what, he could not fire more than anyone else, out of a fully automatic weapon...
so, a new thought on that:
if TUs are the measurement of the speed of a unit and each round has the same duration (in ingame time)...
then anything that could be trained to do it faster would have a fixed TU cost, since TUs get more for an experienced unit and it can do it more often then.
but anything that is limited by pure time or the used equipment (e.g. scanner, non-automatic weapon, etc) would use a percentage based TU cost, so it´d never be possible more often, no matter how fast the unit is.
that may probably be interesting just to see in a test ruleset...
someone did mention that it might do funny stuff with the AI, but since their units can't get any more TUs, it might be easy just to balance the fixed costs on the percentages they´re reserving.
would probably be interesting to have the aliens get increased TU count later in game anyway ^^