Due to the popularity of German infantry doctrine, shows us that grens are too weak to use without 5 man upgrade and it´s benefits.
Consider your opening statement.
What is the most likely scenario; Grens are too weak to use without the 5-man upgrade, or they're simply too strong with it?
If the first case, then the logical conclusion is to make Grens 5-man squads by default - or at least make that a default option for them. However, such a design change would have huge balance implications across the board, and I think is simply undesirable anyway.