Here's how I think a comparison would make sense:
- play something with a looper so you can repeat the identical guitar signal
- demonstrate the sound with the EPSi
- demonstrate the sound with the built-in speaker sim WITHOUT EQ
- then with the EQ used to tweak the speaker sim as good as possible

This would make clear how close you can get with the built-in stuff. And then I could easily decide if I think the difference that remains is worth the effort.
I know this involves quite some work - and of course I cannot urge anyone to do it. It's just that the EPSi isn't available in Germany, so I would have to order one from the US and not be able to give it back. And that is a risk if I don't know how much improvement it'll bring.
So if anybody would be willing to make a demo (soundcloud would do as well as video), I would be more than thankful!