Pole-arm question

You'd need about a 10-15 attack difference or more to make up for lost DPS. It's hard to say without really specific information (such as your stats), but I don't want to do all that math. 10 is a good general number to work with from personal experience from my polearm days. I'd personally keep the Karstan until you can get a 110/120 weapon or an Arcglaive.