It depends on how long of a cut...or should I say, how steady are your hands?:p
When I install a kitchen with a granite or marble countertop, I ALWAYS have a company come out to install their pre-measured pre-cut countertop. but when they come out, they usually have to do a little "massaging" to the backsplashes. They trim them with high powered grinders with special diamond tipped cutting wheels, and then buff them with some sort of a buffing wheel.
If you called any kind of marble and granite company, they will be able to fill you in. Possibly even be able to cut your piece too. For a price........:D
Agreed, have someone do it for you, just a straight cut isn't too hard with an angle grinder and diamond blade, clamp a straightedge down as a guide, but polishing will cost ya a few hundred $ or more just for the diamond pads and you'll need a variable grinder as well, and these will only polish it, not put any type of bullnose on them if that is needed.