Hacksaw. No problem. Take a cm or so first, then more as needed for feel. No sweat.
I didn't want to cut my new $60 grips either!
What you really need are the Ergon grips designed for twist shifters; unfortunately they are not readily available in the USA.
Call Ergon Customer Service. See if they'll swap your right side long grip for a smaller grip, designed for grip shifters. They did this for me, no sweat :-). This will give you what you would have gotten with the Rolhoff model.