iPhoto is starting to show the strain under the number of RAW pictures I'm uploading.
What do you consider to be a "strain"?
Poor response time? I suspect, that will be more due to the network access on a NAS than to the library size.
You might try to put your Aperture 3 library on a NAS, but it is not recommended by Apple, see:
Use locally mounted Mac OS X Extended volumes for your Aperture library
You will have to ensure,
- that you do not open the Library from both your Macs at the same time,
- and you will have to ensure, that the library is only on a disk formatted "Mac OS X Extended". SInce your NAS has a different formatting, put the Aperture library onto a disk image on the NAS. You will have to mount this disk image, before you access the library, like recommended here for iPhoto, see: iPhoto: Sharing libraries among multiple users http://support.apple.com/kb/HT1198
But as I said, if poor performance should be your main reason for switching from iPhoto to Aperture, you will not gain much, if you keep the library on a network attached volume, unless you have a very fast network connection.
The best performance you get in Aperture with the library on the fasted drive you have. I keep my Aperture library on the internal SSD and the original master image files (the bulk of the data) on a second internal drive.
Regards
Léonie