It would depend on the speed of the NFS and the speed of the local network. If your using an object cache like APC the plugins and theme file folders will not be effected because they will be loaded in memory.
A better solution would be to use a CDN or even create your own CDN using a lightweight static file server like Nginx for blogs.dir. W3 Total Cache has built in transparent content delivery network (CDN) integration with Media Library, theme files and WordPress static files.
If you do decide on an NFS there are steps you can take to optimize the performance like the use of NonVolatile RAM, mounting the file system as RAID 0 and making sure you are using UDP instead of TCP.