From what I can tell... upscaling in FCPX is automatic (and rather good!) You may run into trouble when you start adding effects in 8K (or for any upscaled clip of any project size). If you notice the image/clip soften, remove the effects, wrap it in a compound clip and apply the effects to the compound clip instead.
I imagine you may notice differences if the monitor you're watching is actually 8x a regular sized (HD) monitor (27-40 inches). For the most part, I don't think most people are hyper-critical videophiles and would ever even notice the difference, especially on standard sized monitors. However, if your target is a movie theater sized display, then you may have a problem going from 1080 to 8K... You'd probably need to shoot 4K (2K at the very least) to get a decent upscaling.
1080 clip in 8K project: fuzzy tennis ball (no effects/color correction added)

Original clip in a 1080 project (no effects/color correction added):

There IS a difference but you be the judge...