The average effect of inquiry-based teaching is d=0.31, but there are some clear issues that are raised in the (2009) book Visible Learning.
There are many forms of Inquiry teaching, but too often it is implemented without recourse to the content domain. Teaching generic skills of inquiry, which is the common method, leads to this low effect size. Visible Learning promotes the use of SOLO taxonomy. SOLO taxonomy has five levels: no idea, one idea, many ideas, relate the ideas, extend the ideas. The second and third are typically surface and the latter deep processing. Inquiry is more specific to relating and extending (and similar cognitive processes) but it needs ideas to work on. One of our findings is that such processing does not transfer across subject domains which is why Inquiry teaching within (not across) subject domains is likely to have a much greater effect than as a generic method.
A lot of the problems are advocates that promote a method of teaching (e.g., Inquiry teaching) which then takes a life of its own and ultimately the teaching of the content is downplayed. Subject matter vocabulary is critical, knowledge of ideas essential and this too often gets missed when some follow the advocacy of method. Of course, if firmly embedded within a content domain, inquiry is critical. We prefer "problem solving methods" as this directly invokes a notion of "problems - about what".
So, we are not suggesting inquiry based does not work but previous research, based on research using this method, has not been that powerful in its impacts.
Secondly, all work is probability statements. Based on prior implementations it is probably that Inquiry Learning only has a d=0.46 effect. The better situation is asking about your own impact; if you gain d>0.40 using your methods then there is every reason not to change.