are able to shift profits outside of the US through legal entity and transfer pricing structures. But that lower effective rate is not evidence that the high US nominal tax rates are not a problem.

It's an extreme example, but take Microsoft. In 2016, they generated $20 billion of pre-tax income in their foreign entities and a $300K LOSS on their US entity. They paid an overall effective tax rate of ~17%. But it would be lunacy to say our high corporate tax rates aren't a problem with Microsoft because their effective rate is so low. Actually, I guess you could say the high corporate tax rates are not a problem for microsoft -- they're probably all too happy to avoid the high US corporate tax rates altogether -- but it IS a problem for the US treasury that Microsoft can avoid paying our high rates and instead shift their profits and pay taxes in lower tax jurisdictions. At 35%, Microsoft (and every other company) has a huge incentive to do that. At 20-25%, not so much.