IMX, "rewards" for roleplaying don't encourage more or better roleplaying.
those who like to roleplay will, bonus or no, because they find it fun to do so.
Those who don't, who are more there for other reasons like tactical challenge solving or puzzles or social group interaction, won't, because its not what they are there for, whether or not there is a bonus.
if you want more roleplaying, bring more of the former type of player into your gang and less of the latter. tailor your game's scripts and challenges to the former, not the latter and that may very well accomplish this by leading the former to recommend your game and the latter to seek other GMs.
but really, neither type is gonna change their preferences, change their spots, cuz there is an extra skill rank or attribute point or +1 sword in it for their character.