^ That is a Tom Kaye projectile slinger so it's a 'Mag.
A mag'd 'Mag... Let me start by saying I know nothing of these FS rounds amd magfed in paintball. But I do have two thoughts which punisher068 already addressed.
1. Go with a pneu trigger set up as it'll allow you much more freedom in trigger (not frame) relocation and sear activation. However, it'll make tedious tuning even more tricky. Moving the frame rearward seems dubious as it would put the valve-retaining thumbscrew and shim in an inconvenient position. TK has recommended in the past to not use a washerless thumbscrew or standard screw - if you were thinking of going that route.
2. Can't you just build up the height of the mag follower with an epoxy or such to make sure the last round feeds?