The sliders feel the same. I didn't really notice anything different with PBT MX keycaps on MX compatible sliders vs the Topre PBT keycaps on the stock sliders. There's a few issues though.
First, Realforce uses a 6x spacebar, so even if you swap out the sliders, there's no SA 6x spacebar mold.
Second, Realforce might require drilling as well: Topre stabilizers are different from MX stabilizers, so you'll only see one stem on, for example, the right shift vs the three found on MX keycaps.
Note the enter and shift areas on my HHKB pre-drilling:
And now post-drilling with the MX compatible Topre stabilizer:
So... if you're OK with the wrong sized spacebar (which is proof that SP doesn't make the HiPro keycaps) and the compromises associated with it, and want to pick up sliders/stabs, or transplant from another board, you're looking at a pretty hefty price tag. But finding the HiPro keycaps by themselves is extremely difficult, so you'd likely be sourcing a Realforce HiPro, which is an even heftier price tag.
tl;dr It can be done, but expect to pay double what you paid for the board to get your desired look/feel.