40x559 rubino pro is a pretty fast tyre and is cheap at CRC right now.
FWIW I find that 'normal rules for tyre widths' don't apply to frames with 559 wheels, because they are almost invariably about twice as stiff as frames/forks made for larger wheels.
For example, a 32mm tyre width that might be just fine in a 700C frameset can be bone-jarring in a 559 frameset, so a good rule of thumb is to think of a tyre width that would be OK in 700C, then add ~10mm, and that'll be just right in 559.
cheers