Care to point me to some sources on these limitations? I'm just curious.
Not as SPDY as you thought:
http://www.guypo.com/technical/not-as-spdy-as-you-thought/
The guy seems to know what he's talking about, and has actually run some tests but the test setup seems kind of weird.
Effectively he's saying "if you only change HTTP to SPDY and make no other changes, then it's 3% slower (though more secure)" or "if you only change HTTPS to SPDY and make no other changes, then it's only 4% faster" assuming your site looks like some kind of average of the top 500 sites.
And his conclusion is much more positive than most of the 3rd party coverage of it:
"I believe SPDY is still awesome, and is a step in the right direction. This study and its conclusions do not mean we should stop working on SPDY, but rather that we should work on it more, and make it even faster."
The biggest challenge is optimising for SPDY is different to optimising for HTTP and so many of the test sites may have behaviours that aren't optimal for SPDY.
In addition to Google, Facebook and Twitter are/have deploying SPDY so I think we can be sure it has benefits.
He used a non-caching proxy with SPDY, against distributed non-SPDY back-end resources. This is not how anyone in the world will actually use it, and his test is effectively akin to testing the speed of a sports car by having it drive behind a transport truck: The whole foundation of the test is nonsense to begin with.
Got any suggestions on a better way of testing real-world sites?
A number of mainstream sites now support SPDY, so simply turning SPDY on and off on the client side is a simple enough tactic.