If the stock two pipes are 2" each in diameter, that is ~6.282 inches of cross section (A=pi*r2, each pipe is ~3.142 for a total of ~6.282 inches)
The single 2.5" pipe is ~4.912 inches of cross section
The single 3.0" pipe is ~7.072 inches of cross section
I would assume that the single 2.5" would create some back pressure, but I have no background in fluid dynamics for gas so I wouldn't know.