@HarmAssassin there are absolutely merits to the single classed thief (at will invisibility in broad daylight, and *especially* Reliable Expertise), but damage output is not a part of that. Gloomstalker or Fighter 2 for better alpha strike potential, anything 5 for an extra attack, any or all of those will pay off far more than the extra sneak attack damage. After all sneak attack is only once per turn, and it's only 3.5 per 2 levels on average. It's nice for crits and auto-crits don't get me wrong, but an extra 4d6 sneak attack damage, or 14 extra average damage per turn, is unlikely to be better than an extra attack, and especially not the potential for multiple extra attacks (via action surge, Gloomstalker Ambush, Haste, etc). This is even before you consider overkill, total targets eliminated, and so forth.
