You could try flake, or cudaflac. They might be able to squeeze a little more compression out.
I'm interested in making the FLACs as small as possible and have no concern whatsoever for encoding time.
Also, see FLACCL.
Additional settings "-A tukey(0.5) -A flattop" should bring a bit more compression gain.http://www.synthetic-soul.co.uk/comparison/lossless/