You’re assuming all Barr votes would go to McCain. I don’t think you can make that assuption.
Some number would likely go to McCain; some would likely go to Obama. A larger number would probably undervote or stay home.
All in all, it’s not much more than a wash.
There is no way Barr voters would ever vote for Obama. They would most likely stay home if Barr wasn’t on the ballet or hold their noses vote for McCain.
The some number that you talked about would be like 98% of them picking McCain over Obama.