I think a perk that allowed bonus XP would be great.(If there is currency again, make the pro variant give a bonus to that as well) However, I think it should be a perk in ALL three tiers! However, make it a 33% XP increase. If you play TDM, you will get 133XP instead of 100. If you play SnD, you will get 665 instead of 500. That is a pretty respectable increase! However, since it is in all three tiers, that means you can choose which tier you want to give up! You may like Tier 2 and 3, but not much about 1 so go ahead and give that up! If you have all 3, you gain a 99% increase which is close enough to double experience.
I think this is a good perk to those who want to either rank up or forego a perk. If somebody actually has the guts to refuse a perk and opt for something of personal gain instead, why not let him? Nobody can really complain because they only weaken themselves!
Feedback is appreciated.
PS, I can't think of any pro variants if there isn't currency, let alone how you would get the pro anyways.
Seems like a reasonable idea, but the XP bonus has to be significant enough to outweigh the disadvantage of going without the perk, not to mention, the potential XP that you would not earn from the challenges you complete while using the perk.
Using the Ninja perk as an example, assuming you have a silenced weapon unlocked, you could potentially complete all three Pro perk challenges within 150 kills. Since you get 2000XP for each challenge, that's 6000XP, plus you have have the ninja perk. With only 33% more XP, that's only 4950XP (for kills only).
You would need a 40% XP bonus just to break even (again, kills only), but more like 50% to make it worth it IMO.
Of course, if the XP bonus was applied to XP earned from headshots, long shots, paybacks, defends, flag caps, bomb plants/diffuses, etc, the bonus doesn't have to be as high. I would say 40% seems more fair to me if that was the case, plus you would get XP bonuses that are multiples of 5.
However, I guess once you have the all of the Pro perks unlocked and you don't have any perk challenges to work toward, I suppose any amount of extra XP would be nice.