Does anyone know of a good generalized benchmark (either Gartner report or otherwise) for Marketing performance for a private company in North America that is privately owned?

I have worked with garntner report and it has worked great for me. Of course, it would very more helpful if you could provide more details about the company because maybe these sites can fall short to your goals. 

I have worked for several different large privately held organizations and I think the marketing performance metrics will be different based on industry and quite honestly the value of the marketer.  I have always tied my goals/metrics to company KPIs so that we are not perceived as a COST Center.  Not everyone can do that - I realize but if you can it allows you more ability to use metrics that you can benchmark in your industry category ie banking - share of wallet, VOC, NPS, Lead Generation, Brand recognition.  
IT/SaaS is more about Lead Generation/Demand Gen.  Some companies - Marketing is purely about Client experience - retention, satisfaction.  So I think that question can only be answered with more detail about the company and industry vertical of your company.