Bans should be given out with reservations
Otherwise, they will quickly get devalued and people start moving to other places where the price for a ban is higher. You should do something really great to get a ban (let alone a permaban). That's why members looking and striving for power should never be made moderators in the first place since they will heavily abuse their moderator's position (too many bans for too little effort) and severely damage the forum reputation as well (i.e. as a place where different opinions are welcomed)
I don't think so. Bans are not given out for having a different opinion. They are given out only for spamming and that is linked to paid signatures.
A stricter policy might result in more serious members staying, where as the others might leave. Not a bad outcome
As to me, this doesn't look plausible
Why should "more serious members" stay if they see the ban-hammer hitting all over the place? If by "more serious members" you mean more knowledgeable ones, I don't see any such relationship or dependence, I mean between them staying here and stricter policies. Personally, I expect quite the opposite, i.e. such policies would actually revert them from the forum since more knowledgeable people are almost always more tolerant to spamming than to excessive moderation, being mostly of libertarian attitude themselves. I don't like spam and useless posts either but authoritarian and draconian moderation policies I like even less