As far as Fore grips go, I would get a 9" Free Float SR-25 style shroud from Opsgear. Your BT will use the one made for the A5. In addition, I would definitely use a Rip-clip with an X7 hopper. Performance wise I would add a Python compression system to your barrel (Kinda turns it into a Freak Barrel for cheap) & get the Roofus Dawg wicked power tube.
Roofus Dawg Wicked Bolt for BT4:
Rufus Dawg Rufus Dawg BT Wicked Bolt For BT Markers
Python Compression System for A5 / BT4:
A-5 Compression System [A5 System] - $59.95 : Python Paintball, Give Your Balls A Squeeze
SR-25 Style Handguard for A5 / BT4:
Tippmann A5 Floating Hand Guard
BTW, I like the Full stock you are using, How does it hold up to milsim use?