Found this when farting about: http://www.lovehorsepower.com/CFM1.htm

Overlaying my numbers onto a 19t compressor map you can see how efficient the turbo is e.g: 358 CFM at 4000rpm/1.4bar 75% but at 470 CFM 6000rpm/1bar 67% (hotter/less dense air). Interestingly an 18t is very similar - in fact more efficient mid range.

Not sure how accurate the calculator is but kind of interesting if you're planning a turbo purchase.