You can evaluate officials on a range of metrics, such as anonymous surveys on corruption, air pollution, even fertility rates if you want to go for that. GDP targets are silly. Sometimes you have to do tough reforms which will lower GDP growth rates in the short-term to boost it in the long term. For example, China's bloated housing sector. By putting out elevated GDP targets you're undercutting such efforts and local officials are encouraged to forget longer-term issues in favor of short-term GDP targets.