WebP is certainly an improvement over jpg
For small file sizes that's absolutely true.
But I tested out and compared webp some two years ago on higher resolution images and larger file sizes, and I didn't like what webp did to some details and subtle colors. For those I preferred the image quality of jpg above webp.
(but for use within MB that's probably academic)
For the suggested use of caching it might be sensible.
An important factor for possible gain to use it in MB might also be the encoding/decoding speed and cpu use of webp.
I can't say anything on that.
My personal opinion on this one: "if it ain't broke, don't fix it"