Minimal to World Renowned is kinda an unfair comparison because OD if your lucky would be 4-5 weeks per pop with minimal, compared to 1-2 weeks per pop with World-Renowned.
Basically if the guy is 18, and you have world renowned, you will never see 2+ weeks per pope, and most pops with be after 1 week, so yes it does make a big difference between the two.
we noticed that you can bring a player in the ultra-competitive Italy U21 with a level 4 trainer, if starting from a good level. Of course, train single spots every single weeks, minimal or no injures, and 4 seasons perfectly spent.
Level 5 is slightly better, can bring some more pops on primaries over 3 seasons, but at a very high price. I would recommend a level 5 or above only under two conditions:
1) you have a very high potential player and you want to get all out of potential (and you can pay the bill ^^)
2) you must proceed at very high speed in training him
in all other cases, a level 4 is enough. With a level 4, 2 weeks of injuries in 3 seasons, and 4 weeks birole ^^ I brought Scalambrieri to Italy U21, and tool all out of his potential 7 at middle of last season (23.5 yrs). From now on, he will take only birole trainings and I will spend mono to other trainees.