I zapped straight up to Chinese HSK level 4 (like JLPT) which is halfway up the 8 levels of the test, just after 5 months of study from zero Chinese, just because of the fact that I could read Japanese. Then I went into the final year of undergraduate Chinese at university (luckily it was mostly reading and writing, hardly any speaking, in the course I did).
Speaking Chinese wouldn't be much help, but there is a lot of synergy in the written language. I did a semester at Beijing Language and Culture University. There are loads of Japanese over there, and they pick up the written language really quickly, but usually their pronounciation is terrible, due to the limited number of sounds in Japanese.