I’d sort of disagree, but I prefer to take my time in PT1&2 and have the enemies at levels I’m not just stomping all over them. It’s really easy to overlevel in this game and leave enemies in the dust, powerless to damage you.
To answer the question posed in the title, 15% more. If a bad guy usually gives 100xp and you crit-kill him instead, he’ll give you 115xp.
It’s been years since I’ve got to talk about Smirk, I’m the only person I’ve ever seen defend it. I use it as a tool to force yourself to get better at getting critical hits since if you don’t hit crits you’re wasting skill points. For me it was more a mental pressure thing, and I think it worked back then. Can’t say it’s something I’d use anymore, I avoid overleveling and this won’t help me with that.
But if you want to level up faster it should do just that, I’m sure regularly getting 15% more xp would speed that process up. If you’re trying to get through the campaign and get to PT2.5, I don’t know if that’d make that much of a time difference. It still takes time to play through the areas and missions of this game to get you to this point.
If you want to get to max level, I’d suggest respeccing for Eridian Promontory. They’re annoying to crit, but Smirk combined with Trespass should make for some fruitful leveling runs.