I think you have captured all the concerns that I can think of.
More water/depth is good, you can't argue that so long as the balance between emergency overflow and regular drainage is to your liking. I guess the real question is...How often has the water level been 5 inches from going over the emergency spillway?
It would be easier to cut the extension back off if you did not like it. Although, I have to wonder what kind of precautions one would have to consider taking a generator, a welder, and a boat out to do a welded extension? That's a lot of electricity and a lot of water! Curious minds want to know.
I would consider using a rubber coupling and strap-clamps to try out the extension.
