A well fettled (easily done by yourself) S400 does not need a regulator.
The extra benefit of a handful of shots within the sweet spot is outweighed by the cost of fitting a reg.
The resale price of a regged S400 will not be high - zero return on your investment.
In my opinion, a waste of money and just another thing to go wrong.
L/H Pro-Target MK111 Burris Timberline 4.5-14x32 Ballistic Plex
L/H TX200HC Burris Fulfield II 3-9x40
L/H TX200 GinB
Why is dyslexia so difficult to smell?.