[schiefaw]

I understand the problem of the RAM limitation hampering conversion. However, you could try the "Zombie Booth" (among others) method of uploading the document to a server for conversion.

You would need to do something clever to manage the documents since it would not make sense to bring the Visio-formatted docs into OmniGraffle as a normal document. Maybe an email gateway would work in the short-term. At worst, you could release another app that does nothing but handle the communication with the server. The user would select the new app as the "Open in..." option from Mail. The app would send the file up and the converted file would be sent back to the same address. The conversion direction would just be determined by the incoming file type.