I cant talk from the air perspective, but I can talk with similar spec perspective. My iPhoto library is somewhere in the 19.6GB range last time I checked. The only issues I had using my 13" 2012 MBP (dual core, 8GB RAM, HDD) was that I took a good minute to load it all, and when scrolling Id sometimes get the pinwheel cursor. After a while when it fully loads, its manageable. My 15" rMBP (quad core, 8GB RAM, SSD) has no problem whatsoever opening, scrolling, or anything of the sort. Im still unsure if the reason it works a lot better is because of the SSD or the quad core. If its the SSD, I would say the air would be fine, if its the processor, I probably wouldnt go anywhere near the air then.
Hopefully there is an air user with an extensive library that could help you out though.