Yeah, i think it's unfair to double g power, i mean how can you beat that with one ability card? That's probably why they changed the rules.
The character cards never doubled the G-Powers of the Bakugan. That was only on the TV show. The original HSP character cards only doubled the value of the HSP won.
Read my post mister, Doubling your own HSP value wouldn't help you at all, that helps your opponent so then why use a character card? BTW I asked a SM guy before (Seriously asked him, he hangs around Vestroia.com) and he told me that they DO double the G-Power, as to whether or not you still add the bonus I don't know, I'm pretty sure you do though.