Since you have plastic riding the barrel I'm not sure you will get the same results. I've understood the breaking in process to be a burnishing thing, I may be wrong. Now as far as excersizing the action and proving its reliability I would sure be behind that. If the barrel gets broke in so be it. Slug guns can be picky about what they like so you might just run twenty slugs through that bore trying to find what it likes best.
I've had good results with Federal Barnes Expanders, and also with Rem Copper Solids.