So far I'm really happy with the results - a few preliminary tests are showing that playing a CD at seriously excessive volumes inside with both doors and vent fully closed is practically inaudible outside.
Of course the proof is when the drums are in, but I don't think we are going to have any issues.
The only regret I have is I wish I had knocked out the corner where the drums are going to go (see pic below) and built a replacement external wall to 'square off' the room giving much more space. However, it's easy to say these things in hindsight - it very likley would have made the whole project untenable from a budget point of view. The drums will still fit OK, just not as comfortable as it could have been.
Next up is building some absorbers, but I'm going to wait a few weeks before starting that. The good thing is I have almost a whole pallete of the DFM Rocksilk stuff that was used in the build.