I love both of my custom action rifles that I built. One is a Shilen DGR action,and the other is an ARC Nucleus action. A custom action will almost always be better than a factory action in the smoothness that it operates,but that comes with a high price tag. As far as accuracy goes,I can't say that a custom action makes the gun any more accurate. There are far too many variables that fall into play than the action alone. I have several Savage action builds that shoot just as good,and sometimes better than the Custom action's do for a lot less $$$$.

The barrel,trigger and stock/chassis have more to do with accuracy than the action itself.

Barrels are funny though! Two of the best shooting barrels that I've ever owned were made by - Don't Laugh! - E.R.Shaw. Both were 6.5 CM barrels,and shot consistent groups in the .12 -.13 range off a bench with hand loads. I bought several Shaw barrel's on a group buy for cheap,and the worst one- 7mm-08, still shoots 1/2-3/4 moa groups.
I've got pre-fit barrels from Shaw,Shilen,McGowen,X-Caliber,and higher end cut rifled barrels from Bartlein,Kreiger,Rock Creek. They all shoot good,some clean up easier than other's.

Trigger's - There are many choices for custom trigger's with a custom action,I run Trigger Tech trigger's in mine. You are limited on trigger's for the Savage. I like the Accu-Trigger,and have swapped out the springs in mine to the Target spring. On my older Savage actions I have a few with SSS Competition trigger's,and a few with improved 3 screw trigger's.

Stock/Chassis - Since I shoot LH,I have fewer options when it comes to finding these. With my Savage rifles,I have 4 setting in Choate tactical stocks with the aluminum bedding block,and 4 in Boyd's laminate stocks that are pillar bedded. I use KRG chassis on my custom action rifles,a Whiskey 3 and a modified Bravo.
Using a good stock/chassis that is properly bedded makes a big difference to the shooter. If you get something that is ergonomically more comfortable to get behind,and the action sets in properly,it will improve your shooting.

Optics can drive you crazy when you are trying to decide on what to get. I buy the best that I can afford. Pick a scope for the job it will serve,I see too many guy's that over scope their guns. You don't need the Hubble telescope to shoot a couple hundred yards,but then again putting a $50 Barska on a 1000 yard rifle doesn't work either.

In the end,If you have the money to build a Custom rifle,Do it. You will always enjoy it. If not,upgrade your current rifle with good component's and more than likely,your groups will show you the difference.