Here is a modified BSB on youtube with the tunnel filled and note the slide in turns that's perhaps a preferred behavior of the boat for many.
https://www.youtube.com/watch?v=l12QLxWGrdU
Also note all the things immediately wrong with "moving the prop forward an inch" with the mod mounting, servo compartment, and not to mention the negative impact it would have on the rudder's effectiveness.
The flat bottom mod makes the boat much drier a ride for the prop too if you compare videos of the same model without the improved hull. He also enclosed the "bird bath" compartment as was the case with the big swamp buggy I had. No point in leaving anything open on these in the event of a flip which eventually will happen with speed and waves.