Is the harm that the average ethical review board prevents less than the harm that they cause by preventing research from happening? Are principles such as requiring informed consent from all research participants justifiable from an utilitarian perspective?
By "net utilitarian outcome" I'm guessing you mean "overall higher utility in the universe". And I agree, it's higher than some alternate universes that don't contain ethics boards. However, it's probably lower than universes with (competent) utilitarian ethics boards. And the last is probably worse than universes with (competent) utilitarian researchers and no need for ethics boards.
It always depends on what you compare it against.