I work on SparkPost's app dev team - I know you've decided on another route but would love to understand what you were seeing. If you'd be willing to share details with [email protected] or on http://slack.sparkpost.com we'd really appreciate it.

Additionally, we have an awesome deliverability team over here and we work very closely with our users to diagnose and resolve problems like this efficiently. They'd be happy to chat with you about inbox placement should you decide to re-investigate :)