Deltas can fly pretty slow they not all about speed as a matter of fact my first speed plane in many years is a delta/flying wing hybrid. It had an excellent slow speed cruise at 20% power that would give a flight time of about 50 minutes on a 3 cell 3200 mah battery. The speed of a delta is dependent on your AOA and how much gas you give it.
What I was talking about is essentially a delta shaped blimp you would have the best of both worlds where your airframe and payload weight is offset by the gas and you still get the aerodynamic lift as well when you get into thinner air.
The craft you have described sounds like you have a good candidate for what your trying to do.

BTW I am also into pattern as well, not at the competition level, just because I like it...