If it is leagal for you to uprate your rifle to +12 ftlbs then beware that it can NEVER be de-rated to off ticket, so you'll be stuck with it as an FAC and can only sell it on as such. You will only be able to use it on land cleared for same or higher calibers and you will need a slot for the moderator too.
I think the main reason that gunsmiths do this work is because most FAC conversions require a longer barrel than those fitted to legal limit guns.
A gunsmith freind of mine assures me that once you go past 16ftlbs with a standard barrel the groups go to pot as the pellets are stripping through the rifleing instead of engaging it. The longer barrels have a different twist rate.
PS you wont be able to take it plinking at your local FT club either