Something slightly scooped and can chug like a mutherfocker sounds like a description I would give my ceramic nailbomb. I have the 6 string version and its really aggressive and tight. I think it would work really well for 7 string stuff. I also have the alnico nailbomb which is also awesome. Its definitely not scooped, but rather even sounding. Its still aggressive and can chug like no ones business. I like the alnico nailbomb more, but if I played a 7 string or detuned really low I might lean toward the ceramic nailbomb more.
I also have a HD, again mine is 6 string version. I kind of hear what you are talking about, it is very smooth, but works for me for thrash type stuff. I think it might be a little lax on a 7 string though, especially since those are mostly aimed towards heavy rhythm playing whereas the HD is more a lead shred pickup... which is what I wanted. The nailbombs are both great on lead stuff. Ceramic is more aggressive and a little brighter while the alnico is smoother... Both would work I think.