Can I swap Semi-Metallic brake pads for Sintered Metal Brake Pads?
Yes you can do this, and no, you don't need new rotors.

I had the same issue with my CX bike in that it didn't brake as weel as my MTBs, so I swapped to metallic pads.  They do work better, but IMHO, the biggest difference is whether you are using hydraulic or cable disc brakes.  Cable are simply not as good as hydraulic.