graybar says they have independent third party testing off their shelf of their cat6 products performance which they offer. maybe others do as well, i have never had issues with any of the products they have sold me with performance. we have mixed and matched the product lines with no ill effects.
go all cat6 if you go there, and the contractor makes the biggest difference. there is a difference in the performance of the contractor in the performance of the install. those that charge less are usually worth it, in my experience.
exception would be the guy who works out of his home, for himself, independent contractor, who knows his stuff, but has less overhead. his performance reputation is his only marketing tool, and expense.