Re: If it was safe, practical and fully functioning....
A very interesting question. Normally I object to cosmetic surgery because of the wide range of side effects and imperfections, but you are assuming this is perfect so I don't get that easy excuse.
I am in the normal range, so there is no practical reason I would want to be larger. In fact my wife would prefer if I were smaller. So.. would I want to reduce the size of my penis to get more sex? No...and I can't come up with a rational reason why not.
If I were with a woman who wanted larger, then enlargement would make sense. (again assuming it perfect)
Without thinking of a specific partner, I think that deep down I know enlargement is completely silly. Since most men have large enough penises to be functional, it becomes a sort of competition. The end result of that would be guys dragging around elephant size dongs that are too large to allow sex with anyone.
I wonder how many women would want their men to be larger (through the same magic).