I'm not a low vision user, but the (paid) app I've heard most recommended by low vision users for high-powered magnification purposes is VisionAssist by Slinkyware. I've linked you to the AppleVis Directory entry, but you can probably find more discussion about this app. Another good, free magnification app that was mentioned on the Mac-access mailing list is Brighter and Bigger. Here's the App Store link, since this app doesn't seem to have an AppleVis entry, or be mentioned anywhere else on this site.
• Brighter and Bigger (free) by Kazunoru Asada
Both of these are universal apps that are compatible with VoiceOver. I think you may have more color theme/contrast options with VisionAssist, but you really need another low vision user to give you an assessment. Both apps also let you freeze the picture and then magnify further, because holding the camera stable at high magnification levels can be a problem. Hope this helps.