[Rob Babcock]

The shape isn't ideal but that's really irrelevant if you're going to finish the inside.  A dome/sphere would focus all the modes in a really bad way but this could be mitigated by adding some partial walls and using panels to change the shape of the ceiling.  The freestanding dome would have a lot of benefits- it would probably make a great storm shelter and might be enough cheaper to build to pay for the acoustic treatments needed to bring it up to snuff.  Definitely worth looking into.

[JLM]

My old church had a pair of domes.  Strong yes, very expensive to roof right, tons of wasted materials for drywall/plywood, and the matching basements made for lots of nearly useless space (odd shaped rooms).  Adding walls within the shell would just add to the cost and wasted space.

I'd think twice before going that route.  We did, we did conventional stick built after that.