Cheers Ed for the spelling of my question.
I'll take a stab at the question.

I would have to testify. It would be my job to, the whole purpose I was their. Otherwise I might be facing an obstruction to justice charge. I wouldn't want to go to prison for these guys. Why should I? I work for the FBI.
Donnie got too involved and that is what would make it hard to testify. But you have to put things into perspective aswell.
Someone said they would have switched sides. (I forget who) and that would be the easiest thing to do apart from
the whole FBI knows everything about you. No chance, it wouldn't work.
Don't get me wrong. I mean i'm against rat fu**ers. But Donnie, I mean Joe Pistone, wasn't truly mafioso. Pistone hasn't done anything other than his job. However, someone like "The Bull", this is a guy who is a real rat. He should never have done what he did. But after hearing some FBI Tapes he believed Gotti was going to place alot of blame and crimes on him so they played against each other. Sammy came off on top.
Theres no real codes or laws any more with La Cosa Nostra. It's all "look out for number 1".