You can't actually get 20% xp, I tried, the 2 hirelings don't stack. So basically 32ish is the maximum level attainable in the game unless you manage to get respawns, which from the various reports seems to be more of a bug than an actual feature. The achievement is probably from a while ago and they had to cut content for release. If they had like a randomized dungeon with respawning mobs it'd be pretty nice for grinding xp.
As for lost lambs I did the quest with saving them on my 4mages to see. You get nothing. You do lose about 500xp from not killing the bandits though. At the end of the game in the credits there's some little thing where it shows the results of quests where you had a choice(of which there is only 2 anyway, this one and Lord Haart's quest).