Here are three studies on IB:

In Ms. Jackson's abstract of her dissertation she states,
"The results also indicated that no statistically significant difference existed between the average Standards of Learning (SOL) mean scores of Black students who participated in the MYP and Black students who participated in non-MYP schools across the division (p<.05). The results further indicated that no statistically significant differences existed between the average SOL mean scores of Hispanic students who participated in the MYP and Hispanic students who participated in non-MYP schools across the division (p<.05). Lastly, the results indicated that no statistically significant difference existed between the average SOL mean scores of White students who participated in the MYP and White students who participated in non-MYP schools across the division (p<.05)."

Abstract - http://scholar.lib.vt.edu/theses/available/etd-04172006-173212/
Dissertation - http://scholar.lib.vt.edu/theses/available/etd-04172006-173212/unrestricted/DissertationDeborahRJackson.pdf

On page 5 of this Denver Public Schools report it says.
"There is no available evidence that the IB will increase student achievement in DPS schools or that the IB has had a positive effect on student achievement in similar districts or schools. A thorough search of the literature has netted no empirical studies on the effects of IB on student achievement. The IB, itself, publishes no such results."
http://blog.ednewscolorado.org/wp-content/uploads/2009/12/IB-Plan.pdf

From IBO's website, in the conclusion of the article it says,
"On the whole, the researchers of this evaluation study concluded that IB schools score as well as their non‐IB counterparts on standardized assessments of cognitive knowledge and skills, while at the same time providing their students with the added opportunity to develop critical thinking skills from an intercultural perspective." 
There is no attempt to measure, "critical thinking skills from an intercultural perspective."
http://www.ibo.org/research/programmevalidation/documents/TXSum.pdf