bigsby,

You need to find out what the static pressure curve for the fan is, the higher the static pressure available by your fan the more flow you have with elbows & reducers. 2" would work with the proper fan but you would have noise to deal with, I bought a used combustion fan off a furnace that vents thru PVC pipe and it cools my 400W, I ran 4" aluminum flex to the fan inlet and 2" tube exhausting outside.

My fan delivers 60cfm under these conditions, I do not have a carbon scrubber so it I was to hook one up my air flow would go down.