no it isn't easily noticeable, and i never saw some numbers in this direction who seems to be the result of big test(depending that even the training speed is hard to figure out, with all those influecing factors i don#t think that it would come fast).
But i level 5 isn't a bad choiche, at least it isn't a failure who throw you behind when you have solid trainees.
PS: My personal approach is a level 4 trainer with fitness speciality, who are cheap in comparision to the level 5 guys and through the lower stamina skilldowns you also get some "extra" training time - i don't know if this works out for normal teams(i don't have real trainees anymore) too but i am feeling good with this solution.