I just generated some statistics from Go4Go database. As of today, excluding those few games ending with no results, there are
11256 games with 5.5 komi, of which 5961 (52.96%) won by black and 5295 (47.04%) won by white.
24728 games with 6.5 komi, of which 12507 (50.58%) won by black and 12221 (49.42%) won by white.
15010 games with 7.5 komi, of which 7127 (47.48%) won by black and 7883 (52.52%) won by white.
Apparently the smaller 5.5 komi was too easy for black. The increase of komi around year 2003 was a sensible decision. Unfortunately for China, it doesn't make sense to use 6.5 komi under Chinese counting rule. But 7.5 komi clearly gives white a big advantage. Indeed many top Chinese players (e.g. Ke Jie) prefer to use white.