In series the output is a bit more than a Full Shred, but not much. It can sound louder than a FS, though, depending how you EQ your amp, what amp it is and what speakers you have. The Stag Mag in series has a little more bass thump than the FS and the treble can jump out a little more too, so if you have your amp set to really push a lot of bass and treble with a clean tone then the Stag Mag will sound louder than the FS, but if you have a more even clean tone then they'll end up sounding about the same volume. Under gain the FS sounds a bit hotter but not by much.
You have to be careful with the Stag Mag, though, since it's putting a much stronger magnetic field on the strings. The overall magnetic power isn't much stronger than a typical A5 bar magnet humbucker (like the FS and JB), but since the Stag Mag's magnets run right to the top of the pickup, the field is higher and closer to the strings. This is what gives the slightly compressed, sharp 'attack' to single notes, a signature of the single coil sound, but since there are two sets of magnets it also pulls on the strings much more than any normal humbucker or single coil and that can cause a severe loss of sustain or even pull strings out of tune. This is why they don't recommend the Stag Mag for the neck position, where the effects of magnetic pull on the strings is much greater; it's almost impossible to put a pickup with this magnet arrangement in the neck position without sending the strings crazy. For this reason it's a good idea to lower the Stag Mag further from the strings than you would with regular humbuckers, which also means the output becomes lower and the extremes of the bass and treble is rolled off as well.
And as others have said, in split mode it's nothing like any particular SD single coil, it's just a generic single coil tone. If I was pushed I'd say it was slightly closer to a Tele neck pickup than a Strat single or a Tele bridge single, but not by much. That's just the nature of the coil being very slightly smaller than a Strat's and the A2 magnets being quite a soft-sounding magnet. It's got no particular 'twang', nor much 'bell' or 'quack'... just very non-specific single coil. Personally I've enjoyed it more just as a Wide Range-esque series humbucker more than I liked the split or parallel sounds.