Deliverability depends on your list quality and sender reputation. SPF and DKIM help in the sense that you authorized Marketo's IPs to send on your behalf, thus the receiving server can trust this is a legitimate message from your domain. In theory,this can increase your reputation.
BUT, if you are on a blacklisted IP or your domain is blacklisted or you keep sending to bad email addresses, your deliverability will go down regardless of DKIM.
Can you investigate more on the invalid and bounced reason codes? Are you sure that certain emails are valid?
Can you verify SPF and DKIM are setup properly with mxtoolbox.com and by looking at Show Original in Gmail?