Prior Apple Genius here. Bseitz brings up what seems like it should be the truth. The reality is though that OS X and Aperture are extremely intelligent when it comes to allocating RAM. Aperture won't use what it really needs at the expense of other processes to an extent. It knows that you may flip into something more important, so it won't take what it really wants.
Swap used in activity monitor gives a better indication of whether or not you've maxed out your RAM.
Finally, as someone who has used a MacBook Pro with both 8GB and 16GB of RAM in a similar use case to yours, I noticed a big increase going to 16GB of RAM. Going from 16GB to 32GB in my iMac wasn't as big of a difference, still a nice little boost though.
At the risk of hijacking the thread, only doing this here hoping for benefit to more people than myself:
I always get confused about Page ins / Page outs / Swap. I used to use Page outs as my indication for needing more RAM, lately, haven't had a problem with it though. I mostly used Page outs because it also gives a bytes/sec readout, so you can tell what you're currently using. For swap, am I correct that it shows what you've used since the machine last restarted? For instance, my swap currently shows 2.92 GB, Page outs 2.42 GB (0B/sec), and Page ins 24.06GB (0B/sec). 3.42GB available RAM. So currently, I have plenty available, but given that I haven't restarted in a few weeks, at some point I did something that required some swap, and that's still showing up on activity monitor.
Thoughts? Thanks for your help!