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My N has this issue, too. But I'm not really bothered.
Usually the upper gears are synchronized by a single synchronizer ring (due to a progressive gearing, the increment between to consecutive gears is getting smaller with higher gears. Therefor the rpm gap, that has to be synchronized in the upper gears, isn't as big as in the lower gears).
So u might need a little higher force to engage 5th and 6th gear compared to the lower gears which are synchronized by multiple synchronizer rings. This also results in a more direct feeling in the single synchronized gears - means it sometimes feels a bit rough to pass the blocker ring and to snap into the undercut in the engagement ring.
All the crisp transmissions I've been driving so far (MX-5 ND, Mini R57 JCW and now the i30N) had that sort of "problem" in the higher gears. Usually it gets better with time due to wear in blocker, synchronizer and engagement ring...