I've not seen any big difference in the accuracy new rimless bottleneck brass produces compared to what properly full length sized once fired cases produces. As long as the case neck's well centered on the case shoulder, they all center perfectly in the chamber when fired. Both new cases and full length sized ones. Doesn't matter how much clearance there is around the case body when the round's chamberd. And a good full length sizing die without an expander ball centers the case neck better on the case shoulder; the case body's held in place as its neck gets sized down.
Same for belted cases. Belted cases have one other issue; the ridge that appears right in front of the belt after firing a new case has to be sized down to the rest of the case body diameter else it'll interfere with the chamber at that point. Conventional full length sizing dies don't size it down; a special body die has to be used. Innovative Technologies - Reloading Equipment
has a collet die that works fine. But full length size belted cases so they headspace in your chamber on their shoulder, not the belt and best accuracy will be at hand. Otherwise, new belted cases can shoot just as accurate.
The back end of a round's never centered in the chamber anyway; it's pushed against the chamber wall by the extractor in virtually all rifles. So it's a thousandth or so off center when fired. Up front on cases headspacing on their shoulder, they'll automatically center perfectly in the chamber (their shoulder's a good match for the chamber shoulder, isn't it?) as the firing pin drives them there; clearance around the body's not an issue.
If all this weren't reality, then the benchresters would not have switched over to full length sizing their fired cases a few years ago. The size of their smallest groups didn't get any better. But the size of their largest ones are now smaller. Overall accuracy improved quite a bit.