so the fact that it's not in the report doesn't really help you. In fact, if this was THE CAUSE as you claim, then don't you think it would be mentioned at all?
And basic analysis proves you wrong. Only a quarter of Americans are even covered by HMOs. Furthermore, HMO's are a cost containment plan, so that flies in the face of the idea that they would cost more. There's a reason they're almost always the cheapest option on a company's insurance plan choices, with the exception of newer high deductible plans.
The best I can find in your favor is a study that indicates that they don't reduce expenditures by much.