Re: Single coils for metal?
The 2 main issues with single coils for metal are that (1) noise - high gain brings out 60hz hum even more, and (2) most single coils aren't designed to stay coherent under high gain. The type of metal might matter - if you're doing sludge, you might like loose and messy low end, but if you're doing technical stuff, your average stock Strat or Tele bridge pickup won't get far.
That said, not all single coils are the same. There is a rich tradition of P90's in metal (Sabbath, Goatsnake, Corrosion of Conformity), and the Quarter Pound can get heavy (Ritchie Blackmore, one guy here said he uses it for sludge).
EMGs being mostly high output and low noise, they don't represent "single coils" in the average sense.