I might check what I'll be paying for a system from the extractors back....if it's not too much I just go 2" and be done with it, if it's more than a few hundred I'll get into some standard obsessive research on the topic.

Sounds like around 2" if going to be about right anyway - I'll gradually improve the output of this engine from it's currently stock state so a bit of expandability might be a good thing.