It makes sense. I'm not going to do all the math here, so I'll just BS some numbers.
Say that a 363mm barrel has a volume of 10cc's of air, and your piston has a volume of 15cc's. If you double the length of your barrel you then have 20cc's in your barrel, but your piston can only provide 15cc's of that, meaning that the bb will begin to loose velocity before it gets a chance to exit the barrel.
NOW
In doing that I assume that the current cylinder only has enough air for the current barrel, which could be a false assumption, so I could be wrong in that aspect.