Anyone done custom work to there daystate huntsman?

I made a custom stock for one a while ago but that's it.

Personally, I wouldn't change the barrel on an air rifle unless it had accuracy issues, had been damaged or I found a new one that was guaranteed to be better. 

Daystate guns are typically accurate. There is certainly no guarantee that replacing the barrel with a ST will be an improvement over the LW and it could very easily be worse. 

There are people here who have had better luck with both ST and LW barrels but that's how barrels are. There are good barrels and ones that are just average with all of the top barrel makers. When you get one that performs below expectations there are things a good tuner can do to improve them (like polishing and straightening). 

As for the reg, they are certainly available for the Huntsman. I would look at a string to see if it needs it first though. I have seen some truly impressive consistency from those guns out of the box. 

You have a nice rifle there. What is making you think it needs some mods?