I used a length of aquarium tubing taped to a piece of rod for stiffness. Put in a drill bit that was snug in the tubing, and rotated by hand. A bit wobbly but no matter as just knocking off the sharp edge. I followed this up with a dremel stone in the same tube for a bit of smoothness.
Cleaned up TP port, and .125 size still got me 16fpe, so I don't think there is much to be gained by a mirror smooth TP.
It seems as if I have read this in regards to porting 2 stroke engines also as I was doing an RD350. Biggest areas of concern were said to be sharp edges at castings.