Multiplatform games will simply run better on the PS4, it's objectively a more powerful machine
the framerate issues... when have we had a huge console framerate issue in all honesty? I can think of a couple off the top of my head with slowdown when you were REALLY pushing it (Dynasty Warriors games with 60 enemies on screen is my first thought).
you're assuming that a better machine will run multiplatform games better, which is very hard to prove at the moment, especially with how game developers act when creating multiplatform games.
You are right as far as multiplatform games being up to the developer that's why I'm taking the wait and see approach I assumed ps3 would be better and I was wrong I guess we all will find out soon