I put a Knight crosshair front sight on my Colt SP1, as it is easier on my eyes.  I think it would be similar to a globe sight.  When I lower it as far as it will go between the front sight posts, my zero is off from the normal position for the rear sight.  I can zero it fine when I raise both the front and rear sights, but then I run out of available thread on the rear sight to raise it for longer distances.  Is there a rear sight assembly out there with longer threads that would allow me to raise the rear sight aperture higher?

Have you tried filing down the sides of the front sight a little bit? This'll help you to get it to go lower in the tower, and is probably your only way to fix it while keeping your current front sight; I don't think there's anything like you describe available for the rear sight.

Thanks Zardoz, I had a similar suggestion from staff at Bushmaster but thought I'd look for options before putting a file to it.