The reason that Dave Swarbrick has such a great reputation I think is because his pipes are just so nicely made and he has been supplying race pipes for years for a huge range of bikes.
I have used Swarbricks pipes on a couple of TZ motors and an RD400 and I like them. I have no idea who works out the dimensions for him, but I think Dave is the fabricator and someone else works out the sizes, but I could be wrong about that.
BTW, there's no such thing as "just a megaphone" ;-)