Today Visual Office was officially approved to be listed in the Slack app store: https://slack.com/apps/A011VMZC03W-visual-office
The process was a fairly involved one, with their app checklist extremely involved. It was simultaneously tedious but also quite helpful and their response time was excellent (turned around in under 7 days for two reviews).
I'm not sure how much of a boost I should expect marketing wise from the Slack App Directory, but seemed like an obvious first step.