I agree. The TB's with the knob that adjusts the input cap value (I think that's the usual method) can go from the full TB range, all the way to some great thick sounding fuzz tones.
For the OP: I would think of any kind of TB akin to a rangemaster etc. as adding a fuzz to your rig. That doesn't mean it won't get the desired effect, but the feel, attack, and plain old added gain will change things up. I think it changes a dirty amp in a very positive way, but if you're doing tight chugs and stuff, it may or may not be a positive change for you.
I've never played a Naga Viper, but my impression is it's kind of a TB with some other circuit refinement added to kind of do the whole TB>amp thing all in one. Anybody with hands on experience please correct me if I'm wrong.
My takeaway from just listening to clips is if you already have a good amp dirt sound going, a TB with the cap adjustment knob would be what I would choose.
One more consideration, if you're doing the 7 string thing, don't know if you're using any active PUs, but I'd definitely look into how a more vintage style TB will interact with the pickups you're using. In that respect the Naga Viper may have an input that is more catered to actives, buffers etc. I don't remember specifically, but the NV seems like a product CB would design to avoid some of the picky aspects of vintage fuzz designs.
There are no steadfast rules with this stuff. I recently played one of my active guitars with a germanium tone bender, full expecting it to sound like crap. Sounded great, no crazy active buffer artifacts to be had LOL.
Last, not to oversimplify and/or assume you haven't already tried this, but I think you could accomplish what you described with a tubesceamer or similar. Volume knob up, tone up to taste for the amount of high end you want to introduce, probably won't need much on the gain knob. It will also tighten some low end, which will give more perception of high end addition, and will for sure add some "cut through."