I've got a room of this size for drums aswell. My one is full of diffusers, but I'm still not able to get good results with my room mic. So, what I use to is, I mic up my drumkit as usual (OH, Kick, Sn, Toms etc.) and I place one mic (I use a ECM8000 for it) about 2 meters in front of the drumkit. I also use a micscreen on that on to reduce the reflection of the room a bit. Later I put a convulsion reverb with an impule of a good sounding recording room on it (Altiverb got a awesome library for that application). Mix to Wet, cut at 100Hz and 8-9KHz and compress it like hell. Use a really fast compressor like a 1176 or a distressor with shortest attack and release times.