Hi and welcome
1: Playmaker doesn't really impact on games performances in terms of pur benchmark ( or so little that really it's irrelevant). What can really hurt your game performance is poor design, that is not related to wether you use scripts or playmaker. Does that make sense? for example if during your game loop you are doing too many unnecessary things or bad things that Unity itself do not recommend or is affecting performances, whether you do it in c# or playmaker the performance will be poor. PLaymaker will not tell you when you do something that affect performances, you will still be responsible for learning how to build efficient real time applications in Unity, that means read the docs and the various content online to get a feel for a particular feature and it's common practices.
2: Pretty much yes, tho c# access to third party and some corners of the unity api will be more direct then in playmaker, in playmaker, you will sometimes need to create your own actions to fit your needs, but that's already advanced stuff, and the forum is always here for you to requests actions and help on that matter. If you have already c# experience, then no limits!!
3: As far as I am concerned, I know how to script, yet playmaker still saves me time, because it's not just a tool for people not acquainted with scripting, but it's also a very powerful finite state machine which in itself is way easier and powerful than what you could achieve in c# if you have to do event based coding... anytime!
4: I am building professional applications in scientific and technical fields, not games, and they do use Playmaker extensivly. If you check on the member schowcase, you'll find plenty of very interesting games:
http://hutonggames.com/playmakerforum/index.php?board=10.05: Once you buy playmaker, you do not need to add credits to it, it's totally up to you. What you can not do is redistrubite the actual playmaker sources themselves, so if you are two or more developers, each of them will have to buy their own license.
6: I don't know for how long it is on sales, sorry
bye,
Jean