Do they boost the experience gained from quests too? I see that it justs says "combat experience" , so I am guessing its only experience from monsters?
They add more bonus only from mobs, but a 100% potion does not give you double XP XP Potions are useless in BoZ, because you level from quest rewards
Yeah, XP potions are kind of useless after level 100, when the game requires you to complete quests to level up. It just become impractical to try and grind unless you want to spend 4 years working on one level. Save them for the hell levels of 95 to 100. Those actually do take a lot more work, but grinding is possible.