A team here at eHealth Africa built a custom data collection app for disease surveillance communicates with DHIS2. They included SMS submission, and they told me that the single biggest improvement they made was to add compression. They found that this reduced the number of SMSs per form submission from 6-7 down to 1-2. This meant that the submission success rate increased dramatically, since there were fewer possible points of failure (the app is used in Sierra Leone, where cellular connectivity is pretty unreliable, especially at remote health facilities).
We've just open sourced the library we developed to do that, so maybe it could be the basis for an ODK-specific implementation?
(Documentation will be improved in the next week or so, I hope!)