OK, so I've tried a few attacks using only 4 dragons, and 60 minions... yes it's quicker to brew, but I beleive it's a lot weaker - I struggled to get a 1-star victory on a couple of TH9 bases that I was aiming for a 2-star on, and I'm currently only around 2260 trophies. In particular:
- It's really hard to reach air defenses that are in the middle layer (not the centre, just betwwen the edge and the centre).
- The minions invariably clump up around a wizard tower, and you lose a bunch at once.
Having 6 dragons, you have 6 tanks who can take plenty of damage before they die, and you use rage if and when required to move them forward quickly to take out air defense, high level xbows/tessla, etc. More than 6 would probably be stronger again, but it's very expensive in terms of elixer.
So I'm sticking with 6xD, 40xM.