Would a better way to do this be to have a different root element.
<index> say? (I have no idea about the XML conventions for roots, so
forgive me if this is a really bad idea).
The other problem that often isn't discussed is: Where do the feed
level elements go? Well how about to be a link to a standard feed
document (with modified date/hash, obviously)? And it follows from that
the feed document might have entries of its own. And then you can set
up SSFF as an alternative on top of standard Atom. Basically the
consumer starts by subscribing to the full channel, discovers there's
an SSFF feed available and switches to checking that. Simpler consumers
won't need to do this.
(That last bit is mostly there to satisfy people who don't want to
complicate how much a minimal implementation has to deal with - but as
it'll be up to the publisher to choose how many items/what metadata is
present in the Classic Atom feed - perhaps not much - so it might not
work out that way)