The simple formula is: ROD (fpm) = % Gradient X Ground Speed (Knots)
Minor side note for completeness. While the formula cited is used generally and is fine for pragmatic purposes, when one runs the unit conversions, there is around a 1 % error involved. This, normally, is ignored. However, just as well to be aware of the consideration.
Regarding varying the cat, I am rather amazed that folks might want to reduce cat limits with a weight reduction. I was always quite happy to adopt a higher cat on the jets (especially for nastier aerodromes) as that made everything a lot more comfortable ... just meant we didn't get in on occasion. Cost of doing business in my simple view of life.