Eliminating the plastic behind the grip not required for the main housing and relocating the batteries as you suggest would actually make this blaster more unwieldy. Unlike the Vulcan, which was extremely rear heavy, the Stampede is well balanced. Moving the batteries anywhere else would alter the weight distribution of this blaster, which I was told they researched thoroughly. They learned a lot of lessons from the Vulcan. The battery pack doesn't seem to be a power sled like the Vulcan was either. The battery chambers are the back of the blaster behind the grip, and the travel range for the plunger system also goes back there, so I don't think you will be able to remove or relocate anything without negating the benefits of this blaster design.
cut it off just forward of where the darts clear the clip/breach, eliminate all plastic behind the grip not required for the main housing, relocate the batteries (on gun/off gun your pref) and i forsee mad dual wield pistol possibilities.
I have *never* understood why anyone would create a long 'barrel' for nerf darts when the get their force from leaving the chamber.
As for removing the cosmetic barrel at the front, there isn't much you can remove without also cutting off tactical rails. If you lose the bottom one, you have nowhere to mount the combination bipod/grip. Cut back further and you lose the rail that the shield is mounted upon.
I don't think simple "hack and slash" modifications will benefit you on this one. If you want to modify it, it will take some more thought and precision, like the RFSG did to produce the Doomsayer.
Someone will figure it out, for sure.
The ultimate "pistol" minimization would require the removal of the batteries altogether. Perhaps replacing them with RC car batteries that are worn as a pack, rather than included in the body of the blaster.