There is no point at which the xp return makes something strictly or “mostly” up-side.
Before level 15 it is 100% at all times strictly beneficial: a player should ALWAYS have maxed out bonus XP before max level, even if all they are doing is flying up to pirate fleets and then running away for 100% bonus XP (though there are so much better uses than that).
Any argument about it being wasted time only applies at max level. Even then, there is a tradeoff between time and value, because the S mod gives value. In this case: a new officer. If you want to think of it in terms of return on investment: there is an initial period where an S point has been traded for an officer (negating the need to find one). Then after enough fighting to earn what would be 1 S point, they have recouped ALL the initial investment, but still have the officer.
I think it's precisely the opposite. Any argument that you get a story point back from he bonus XP only applies AFTER max level. You get no benefit from additional bonus xp until you exhaust your existing bonus xp. So effecitively, there is NO benefit from the 100% bonus xp
until whenever you run out (which is usually super late game in my experience).
You will be in exactly the same situation minus one story point until super late game when you have already finished creating your fleet/officers, at which point it is much less important IMO. You are effectively delaying an S-mod or elite skill until super late game to save 2k credits on an officer, which is a terrible trade IMO. I never spend any story points on things that I can get in other ways. I pretty much only get s-mods, elite skills, and officer mentoring (no other way to reliably get the right combos of skills and aggression) and that is enough to maintain bonus xp until well after max level. As long as I have bonus xp, spending story points on anything else is just delaying me getting those benefits that I cannot get in other ways.
The only time I would even consider promoting is if I ran out of bonus XP and didn't have a single s-mod I thought would be useful long term
and I was having trouble finding officers (which is usually not an issue for me).