Properly run slicetype frame cost with 2pass + MB-tree.
Slash the VBV rate tolerance in 2-pass mode; increasing it made sense for the highly reactive 1-pass VBV algorithm, but not for 2-pass.
2-pass's planned frame sizes are guaranteed to be reasonable, since they are based on a real first pass, while 1-pass's, based on lookahead SATD, cannot always be trusted.