Actually, yeah. I used to mod over at sevenstring.org and co-founded metalguitarist.org after we sold the site, and have played seven strings for about a solid decade now. I'm more in the shred/prog world than pure metal, but I'm pretty familiar with how people use them in a modern metal context.
Myself, I play an Ibanez Universe in B standard, strung with an Elixir 10-48 set with a .68 for the low B. I hit pretty hard, a relic of my blues-rock roots, and these gauges hold up to a heavy picking hand pretty well. I plug into a Mesa Roadster, generally straight on in.
On the string gauge thing, opinion is split. A lot of guys like really light strings for low tunings - they tend to be brighter and less compressed than heavier strings, which for rigs with a more compressed response tends to work pretty well. Dino from Fear Factory is a name who gets tossed around here, tuning to Bb or A with something I'd personally find almost laughably light.
The other side of the coin is guys who play through rigs that reward you for hitting pretty hard and not relying on a ton of preamp saturation - Mesa Rectos are pretty lightly dampened in terms of response, and sound their best if you don't jack the preamp gain all the way up. Guys getting really good sound out of Rectos tend to run heavier strings and lower preamp gains, while the lighter string group tend to prefer more saturated preamps. It's a matter of taste, but the real thing is making sure that the amount of compression in your amp compliments how compressed your guitar sounds acoustically (heavy strings have a more compressed attack due to the added mass, which gives the illusion of more "sustain," while lighter strings are less massive and a lot snappier). As long as the pieces work together, you should be able to get something to work.
Sounds like you tend to see eye to eye with me on string tension, in that I've always felt that most "standard" sets are too light. GHS Boomers' 9-59 set is about the best-balanced stock one I've found, but I'd recommend just buying a six-string set you like and then ordering an extra 7th. If you're in the states, I've had good luck with Juststrings.com.
Another thing you'll see a lot is guys running a Tube Screamer in front of their amps. This is less for the gain boost it could provide than it is as sort of a "pre-EQ" - a seven string has a lot of low end, so using a Tube Screamer both changes up the balance of even and odd harmonics (which can be nice - again, taste) and tightens up the low end a bit, and has the effect of focusing the "range" in which the preamp tubes are driven a bit more. For really tight, saturated, focused guitar sounds, this is the way to go. For slightly lower gain, bigger, more growly sounds, a boost might not be the answer. I tend to prefer my Recto unboosted, but your mileage may vary. One way or the other, this is something that's kind of become associated with Andy Sneap, so if you like his work this is the way to go.
After that, really, it's just common sense, and it's not much different from getting good metal tone from a six string. A guitar is a midrange instrument, so don't scoop your amp to death. Especially with a seven, you probably want to be extra-careful with your bass knob to not stomp all over the bassist and drummer. Ditto with the gain knob - there's of course more than one way to get a great sound, but I tend to think you get "bigger" sounding guitars by focusing on getting a good attack from your tone, which means being careful with the gain and low end and letting the guitar really punch through the mix.
