I think one of the biggest problem here (and in general too) is that when your opponents, especially the top 2-3 teams that are actually giving the "strength" of your group have strong starters or much more stronger line ups.
In this case based on my experiences you will not be able to beat them with a balanced team.
In a normal set up their starters are playing 38 - 42 minutes / game (depends on the score, strategy, etc), hence in the remaining few minutes that a substitutions are getting you can not change the game result.
What I have observed so far is, the majority of the game will be slightly or heavily mismatched having a 30-40k roster against a 70-80k roster and you will not have enough time to change the scale in such a short time that your subs are getting.
On the other hand, having only a 6-7 massive players rather than a balanced team has always the risk for one injury can be a disaster.
Sz.