Yeah but if you're only putting 8 GB of RAM on then you're also going to be constantly querying the hard drive. So any performance gain you get from soldering, is lost by going all the way to the hard drive every 3 microseconds.
It's only better performance on paper in reality there's no real benefit. If you can run an application entirely entirely within the 8 GB of RAM, and assuming you're not running anything else, then maybe you get better performance.