What I have found so far in regards to cleaning the Attack - The factory provided cleaning set up is a lever mechanism that moves the turbulators up and down in the exchanger tubes. I do this before every burn, takes about 5 seconds and nothing needs to be disassembled, just grab the lever and move up/down 4-6 times. Now a brush run thru the tubes would get them cleaner and I have done that once this season. I keep an eye on exhaust temps and so far they stay pretty consistent and they did not drop much when I did the brush cleaning. If they start to creep up during burns that is an indicator that a better cleaning could be required. So far it does not appear there would be a big efficiency advantage to brush cleaning the tubes more often. The Varms are by far the simplest to brush clean the tubes.
If I thought there was a big advantage to brushing the tubes on the Attack more frequently I would modify the lever mechanism to make it easier to remove the turbulators - as it is with the factory set up it's about a 30 minute job because the part of the lever mechanism must be removed to get the turbulators out. Others have done something similar with EKO boilers as a lot of the Euro boilers have a similar set up. I will agree with
@maple1 on the chimney cleaning, once the gases have gone through the nozzle, there is nothing but light fly ash and chimney cleanings are really not necessary or can be done much less often. (assuming of course you are not trying to stuff a bunch of green wood through it and not gasifying)
Thoughts on wood moisture content. In a forced/induced draft gasifier with storage the boiler is running wide open. IMHO wood dryness is even more important on a non-gasifier be it a boiler with no storage or EPA woodstove or furnace because these are usually run with the inlet air restricted. Now I'm not saying you should run green wood through a gasifier, but I bet it would be more tolerant if there is a piece or two of not optimally seasoned wood in the mix.