I don`t think any of my servoes have suffered from broken gears during flight... And I have flown since -75. Only time gears had been damaged (except from crash) is those connected to steerable wheels. (And when rudder surfaces hit the doorframes on the way from the hobby room to the car...) Thus I now use metal gear servoes for those applications. And I use MG for flaps servoes on my gliders too.
Else I prefer no to use MG on my pattern planes, just because I have a feeling they get more slack than nylon and carbonite gears. And I don`t like slack on my pattern planes
For 3D from 50ccm size and above I also use MG due to the abuse they get during nasty manouvers
15kg for the rudder on a 2m Extra should be sufficient. I think my 2,3m Extra have a single 18kg for rudder, and it keep up in knife edge and knife edge manouvers. A friend of mine did try 10kg for rudder in an equal plane. He felt the knife edge wasn`t firm. Changed to a stronger servo and he got a far better performance in KE.
The 27% Extra is nice
A perfect plane for electric motor!