As the listed upgrades mention plenum size - barrel length, then i am also pretty sure these can be FAC just fine.
Sub 12 is by no means undecidable, but i think FAC would be what most would want.
As long as i am not yet locked to Sub 12 or maybe even less like that, well my money are on FAC,,,,,, and slugs. I am pretty sure if i was allowed to own a larger caliber like say .22 or .25, pellets would be even more interesting for me in those calibers.
But shooting 80 - 100 M with pellets in a .177, that at least with my current experience is too much of a game of chance, but i have seen other shoot .22 and pellets pretty damn fine at 100 yards.