We will output a 1080 signal natively at 1080p30 or you can hardcode the stream to 1080 (Settings - BoxCaster - Edit - Advanced Settings) if you send a 1080i59.97/60 signal. All other 1080 signals will be served out at 720.
These are the frame rates of streaming at the following resolutions.
720p60 input encodes 720p at 30fps
1080p60 input encodes 720p at 30fps
1080p30 input encodes 1080p at 27fps
1080i60 input encodes 1080p at 23fps
Check out this article for more information about Compatible video resolutions/frame rates.