Hi Howard,
We did test live view in a real world low light scenario, and were very impressed with results. This test will be published soon and covered in depth. As Steve mentioned, the outdoor live view testing is forthcoming, but from early results using live view in highlighted areas appears to be much less of a, if not entirely eliminated, issue.
As Steve mentioned, we do not have a Sony A7R readily available to compare with, but after sampling a 5D Mk III's live view capabilities through the camera body as well as through Capture One software, I must say that the IQ 250 stacks up surprisingly well. The large sensor of the IQ 250 does render the live view somewhat slower than the 5D Mark III, but negligibly so in terms of active use.
As on other IQ backs, it is possible to use Live View to check up to 400% magnification, and with little to no sacrifice in performance/refresh rate.
Hope this info helps!