rraffo - good questions.
1) No. LFJS is not a replacement for the standard physics plugin. It is just an alternative with more/different features - especially comprehensive joints and particles from liquidfun.
2) I very tentatively say yes. There are more features to LFJS, but how easy it is to replace one with the other totally depends on how many events you have used in your project. The general design is similar, so actions should be transferable with minimum fuss (unless you have 1000 of them...). The functions and events do not all have the same names and will not automatically swap, so it will be an event by event process.
3) The standard physics engine is very fast, but the implementation is not complete in C3. LFJS has similar performance to the standard plugin (based on a similar library) but there are a few more lines of code in the collision presolve callbacks. So, techincally, I expect that, if you were to enable collision points in LFJS, then LFJS might ever-so slightly under-perform compared to the standard plugin. There is a stress test demo that compares a tower of blocks in LFJS to the standard physics plugin and, to my humble i5 laptop, there's hardly anything between them (I think the standard plugin takes the edge, only just).
All questions are welcome. :)