I know 2 trouble about disc usage:
1) plesk8 didn't count log file sizes. This was fixed in hotfix-80060713 (psa-hotfix2-8.0.1).
2) plesk8 fails on counting disk usage for directories with more than ~ 10000 files inside. This was fixed today, plesk support send me new /usr/local/psa/admin/sbin/statistics file.