What to choose for metal and lower tuned stuff

[JDC]

String gauge/tension is going to make a difference in relation to the scale length too, ie on the lowest string, longer scale lets you use a thinner string while keeping the tension so you maintain the clarity while a shorter scale will need a thicker string so there will be more beef.

Longer scale will also make the higher strings harder to bend ie more tension so it will feel closer to bending 10s instead of 9s (google string tension calculator if you want to get nerdy about it ;) )

If you want to play in C/C# I don't see the point of uptuning a 7, if you want the 7 you need to ask yourself if you will really use the upper strings

I have an agile intrepid 8 (28.6"), don't know what your preference is for necks but for widdley shredding like there is no tomorrow I personally think it's 2mm too thick and the body needs more contouring for my picking hand, wish it had a locking nut to make bending slightly easier

[Sifu Ben]

I've got an SG in C# with a 10-52 set, not a problem, and I've got a 27" 7 string (Agile) in B standard and it doesn't really require much adjustment.

[Alex]

It is a lot a matter of taste and feel. 7-string or baritone are really different animals. I love the fact that the 6-string baritone I have is not as cramped and has a naturally wide neck, but other playes might prefer 7-string normal scale guitars.

However, some comments to other posters:
I agree with SifuBen that 27" isn't such a huge difference, although it is noticeable and tends to require a bit cleaner and more precise fretting.

I think .56 on a LesPaul might have worked for Bill Steer in Carcass, as Richard pointed out, but IMO the sound on Heartwork is oversaturated and lacks definition, as well a result from the Colin Richardson "wall of sound"-production. Bjoern from In Flames uses a .62 for B flat, but I think his guitar is EQ'd a lot to sound relatively dry and not overly bassy. Each to their own, I'd say. I think it was Kerry King who once said that it depends on the guitar sometimes, some just hold tuning and intonation a lot better even with thinner strings. That might be very true sometimes.

[Lucifuge]

Tony Iommi is known for down tuning on an SG with light gauge strings, so it can definitely be done.