Personally I think for a serious glob of something nothing beats the angle grinder with a DIAMOND blade, not a masonry gringing wheel. Diamond goes thru fired clay and glaze materials like a hot knife through butter. Lots of time working in Japan with wood firing potters showed me the true wisdom of this minor monetary investment. The blades are not that expensive and last a good long while. I've been known to use the same blade to clean up wood fired work and kiln shelves and posts from my noborigama for at least a year.
You'll need a respriator and (impact) eye shield. Prefereably grind it outside where the dust is not such a big issue. Put the shelf on a soft support before grinding.
Using a hammer and chisel as well as using a rough surface masoney wheel puts a lot of vibrational stress on the kiln shelf. If there are any micro-fractures in the shelf structure, this likely will tend to exacerbate those issues. The diamond wheel is very smooth and imparts little vibration.
On corderite and high alumina shelves, as well as kiln posts, the diamond blade can go thru the SHELF or post quickly too. So be careful. Silicon carbide is a bit harder on the Mohs scale.... so the diamond does not eat thru it instantly.... but you have to learn how much pressure is too much there too. (My Advancers resist the diamond pretty well.)
Do you mean those segmented diamond dish (cup) wheels (that sound to me a way too aggressive for this purpose), "regular" (electroplated or rigid impregnated) flat or cupped non-segmented diamond discs, diamond impregnated rubber discs or those diamond pads that get attached to a rubber backer with Velcro? What grit?
Do you happen to have a photo or a link showing the one you use/like?