Removing the deck gun doesn't give you any extra speed or advantage. Agreed that after a point it does pretty much become useless, unless you move into a VIIC/41 which doesn't have the option of using one I generally leave them on just in case your out of torpedos and you've got a stray merchant ablaze like a forest fire and it only needs a little more damage before it goes down, firiing from 2,000-4000m for a few quick shots is usually safe enough even late in the war with just a single ship around and plenty of time to submerge or leave the area.

This place has a lot of great u-boat artwork as well as other areas of interest (tanks, planes), many with original signatures from the u-boat captains:

http://www.u-boat-art.com/