I'm not sure how it works in all states but I know where I am at the local MLS requires a huge questionnaire including hardware specs, firewall specs, how you protect against scrapers etc.. there are probably 10 sites that have the entire MLS hosted on their domain that I could find, NONE of them have pages which are considered supplemental, all pages, and homes in the index are fully indexed with Google atleast.
Hmmm, well I guess you have your answer then. Go ahead and dump 40,000 pages into the index. Let us know how things progress for you.
Did you perform any research on those 10 sites? Are they authorities in their space?
What will probably happen (if you release 40,000 pages at once) is those new pages will get indexed and many will end up in the Supplemental index shortly thereafter. Then you'll have an unnaturally high ratio of pages in the Supplemental index which in turn may have an effect on the overall performance of those pages not in Supplemental.