Mat, my personal opinion re. 18v EMGs and metal is that it can be detrimental depending on what you're going for.
As stated, the headroom is increased which increases the dynamic range, ie. you lose some of that 'compressed' sound that makes them a great metal pickup.
It's easy enough to install a temporary switch to hear both sounds. I'd suggest making the control cavity large enough to add something like an SPC or VMC to beef up the mids should you find that necessary later. Before the 81-7 came out a friend of mine had me add an SPC internally to his 707 equipped axe. I replaced the original pot with a screwdriver adjustable trim pot and mounted it inside with double stick tape. It got removed when the 707 bridge pup was replaced with an 81-7.
Now, what was the moral to this? Oh yeah! Try both options, see what you like best