Just a quick update to say that I've found a fix for the problem.
Searching through sunsolve last night I ran into bug 6709765.
Its description of slow HTTPS performance was exactly the symtom I was having with grizzly and HTTPS streams.
I've applied patch 128647-05 which upgraded my instances to build 08-p03, and the problem has been fixed.
Cheers,
Corey.
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