race cassette conversion st 5

[Petropoliskhan]

Hi, I'd like to know if anyone has any experience swapping out the 11 speed mtb XT cassette for a race cassette (ultegra, 105 or similar) on a Stromer ST5 with an XTR di2 derailleur . The objective being to have smaller steps between gears and being able to maintain my preferred cadence. In my opinion and on my usual routes the 5 largest sprockets are useless. The routes I ride are hilly but not steep enough to require those gears.
Many thanks