That was my theory also.
I was going to make a 3" system with a 2.5" back box as the gasses would have cooled an be traveling slower, thus the reduction in size as it reaches the rear might help maintain velocity.
But unfortunately it seems to be quite confusing as is supposed to have a negative effect on the harmonic and pulse wave characteristics. I maybe thinking to much into it but can't help myself.
Although the standard systems do reduce in size from 2.3" to 2" into the back box. So maybe it is something to try.