(FULL DISCLOSURE: I wrote one of those questions referenced, and suggest including another similar question to consideration.)
If you're going to merge those two, also merge in my other canonical question:
However, I think that both (all three) have a place, and could all be displayed.
If they are merged, in the spirit of being a resource for Professionals, let the opening be "We are not going to help you with your Capacity Planning and Load Testing. Here's Why: (answer regarding your special snowflake goes here)." Then, "However to be helpful, here are some thoughts we've put together on how to go about it." And possibly links to the "Web Operations" and "Art of Capacity Planning" books.
On a side note, I would like to see the Canonical Questions made more visible. Personally I'd like to see it right up next to the Questions link or the Search Bar, but at very least I think that it should be added to the FAQ.