What is the missing sauce? My understanding is that once your sending IP has some history and has developed a reputation as not sending spam, there isn't anything offered by Sendgrid that improves upon that.

If you set up SPF and Domainkeys/DKIM that is an added vote of confidence but Sendgrid doesn't even require it, so I think most of the value of Sendgrid is simply offering a stable IP address and a convenient API. Am I wrong?