The cutting power of lightsabers varies widely, even from scene to scene within a given source. For example, in episode IV, when Obi-Wan faces Darth Vader, Vader's lightsaber quite clearly fails to cut Obi-Wan's robe. And some of the books I've read have had jedi facing robots that die in one hit, and then robots that are so heavily armored that a lightsaber is "useless". Seriously.
As such, I'd tend to say a lightsaber is an Item of Power. You have to have an aspect ("My Father's Lightsaber" or "A More Civilized Weapon" or whatever). It counts as weapon: 4, and has the sword of the cross' "all creatures are equal before god" ability. So it quite literally runs at the power of plot; if you've got a fate point to spare, it's an unstoppable weapon that cuts through whatever is in your path. No fate point? I'm sorry, that heavily armored blast door is just a bit too tough to cut through before the enemy reinforcements show up.
As for it being "not a big deal at submerged or above" - keep in mind here that DFRPG is a higher powered setting than star wars. A star wars jedi typically has inhuman speed, channeling, a lightsaber, guide my hand, and perhaps a few stunts. That's less refresh spent than a starting wizard. Even the big villains of the movies only added a little bit on top of that, upping channeling to evocation, and possibly picking up a few incite emotion based powers.