Hoping someone may be able to advise as I've not had much luck with online chat.  

I have an existing M5 network with 4 units.   One at front of house (Main deco) which links up to the one in the loft and to one at the back of the house.  Then the one at the back of the house links to the Cabin at the end of the garden.  Both the loft and the Cabin could still do with a boost as gaming and video conf can be unstable, so I'm wondering if upgrading a few M5s to M9s will help?  If so, which ones should I replace?  Trying to keep the cost down so don't really want to replace all 4 if possible.

Any advice gratefully received.