I think a 31mm tall saddle pack might be pushing it unless the wiring is all outboard on both packs rather than inboard near the top deck. I have a 5100mAh Protek R/C that is 25mm tall and the plugs are near the top plate for the jump cable and they just barely clear. I have seen on here were guys have gotten around it by wiring into their bullet connectors at a 90-degree angle to add clearance. The other issue might be the length of the battery posts... some guys are just using locktite to hold the threaded portion higher in the blue ano post for more room, but I feel that 28mm is about as tall as you want to go and still have good handling and no contact with the top deck. If I remember right 31mm will fit under the deck just barely, but you might have to make other compromises.