I used a Dremel with the light duty cutting wheel with no problem. If you go to Lowes or Home Depot, you do need to pay attention to the type of sink tube, they are made of brass, but some are 20g and some are 22g, the 22g being thinner will be pretty easy to cut (this on some boxes is the difference between "Consumer" and "Contractor" or "Commercial". Use the Dremel at its highest speed (wear face protection!).
I have also found that as far as drilling holes in sheet metal use a Uni-bit, its a stepped bit shaped like a cone, they come in various sizes-spendy, but worth it, and you will have a nice hole with no burrs. Put a wooden closet pole inside the tube, then a centerpunch to mark your start point for the drill bit.