Hey Arran,
A few concerns cross my mind.
1) The TLM has a cardioid response pattern. If I'm not mistaken, this means you'll miss out on a partial of the acoustic activity.
2) The TLM has a frequency response which emphasizes frequencies 3000 Hz and up.
3) I'm not finding any specifics on the 'Studiospares' speaker you refer to. Is it a custom build?
What is it exactly the graph named "COBC Studio 1 Energy" plots on the horizontal axis? The one with the horizontal scale from 0 to 11.
On the subject of RT60, RT30 and RT20. Reverberation Time (RT) is a measurement of the time lapse between the moment the source is cut off and the moment the reverberation drops by a set amount. This can be any quantity of dB's one deems appropriate, but the ones mentioned above (60dB, 30dB and 20dB) are the ones used most frequently. You can then have your computer analyse and plot the RT for seperate frequency bands, as you've done already in your graph.
Ps. Why have two separate guitars when you can have all in one?