I race small nitro engine boats (almost exclusively wide open). With my 18CVR-MX, although it recommends 10-30% nitro; I normally run 30%-50%. I have just done some experimenting with the 18TZ-TX w/11L which recommends only 5-20% nitro. I have tried my 'normal' 30%-40% with the P8 cold plug and in back-to-back tests actually picked up 1.6mph going from 40% DOWN to 30%! Could this be because of the port timing and head volume? I didn't have any 20% or additional head shims with me or I would have tested them also.

I thought that the higher nitro fuels had more available energy - is that not true? Could having some port timing work done release even more power; if so what kind of timing should I be looking at? Also, couldn't I just add another (or remove one) shim to adjust the ignition a little?

If you improve performance by going down in nitro content, then you're running a fuel that the engine's timed for. Nothing more. If you need to run higher nitro, then you may need to reduce the compression a bit, change the plug, and so forth. There is no one, easy answers to this.