Elastic Search Indexing the Internet
An index per domain (so 275 million indexes) is not feasible. Indexes do have an overhead, and I've lost the reference, but I don't think you want more than ~100 indexes on a single "normal" server.
To get more sites into a single Index, you would want to introduce routing and views, but I would imagine that a single index for everything would also introduce un-needed overhead. I'm guessing, but the routing rule look up might become incredibly large etc. So you would want to find some way of splitting things across indexes. At such a high volume, you can't design it all on paper, so I would advise PoC work to determine what kind of performance you get for different sized indexes. You would then look to use aliases to map correctly to the underlying index.
For further reading:https://groups.google.com/forum/#!searchin/elasticsearch/index$20per$20user/elasticsearch/i-G5NlP1VeY/PK9vVP0myAgJ
https://groups.google.com/forum/#!msg/elasticsearch/9L5cWIAib94/K7zdHEW-4P0J