Interesting results from an experiment I ran recently to switch Google+ from downloading CSS via XHR to using the link tag.
We were using XHR to download CSS (this was to work around an issue in IE - exceeding the limit on the number of rules in a single stylesheet; for convenience we used XHR for all browsers). When it came to our attention that XHRs are requested at low priority (unless they are sync), we decided to run an experiment to see its impact on G+ latency.
In SPDY capable browsers this resulted in a big latency improvement. In Chrome 27 we saw a 4x speedup at the median, and 5x at 25th percentile. In Firefox 21 we saw a 5x speedup at median, and 8x at 25th percentile. No significant impact was seen at the 99th percentile.
In conclusion, in a SPDY world, XHRs are requested at low priority (except sync XHRs), and are opaque to the browser. Using declarative markup like <link> enables the browser to do appropriate resource prioritization.