If shooting FSR's why use full auto ? is it cool, sure, but impractical. With FSR's and a rifled barrel you shouldn't need but a double tap to mark a player (at distance). If your close then a non-rifled barrel should be fine. Granted we hadn't seen a rifled barrel yet for the MR5 but soon will Im sure.
My suggestion is skip the E version and just buy an MR5. Your playing mag-fed and shooting FirstStrikeRounds... so why have full auto ? Even if your running normal paint, its a mag-fed marker, no need for full auto IMHO ;-)