You do not indicate the caliber of the gun, the weight of your pellets, the current velocity, or your expected velocity. You can certainly enlarge the transfer port and doing so MAY result in a velocity increase. If you go too far you will experience a reduction in velocity. You do not mention which spring you are trying for a velocity increase nor what you have done with piston seal sizing if anything. Have you checked your breech seal? There are several avenues to explore before modifying the transfer port. That operation can be really hard to reverse if you go too far.