I have a Polymer80 with a RDS, Brownells slide and lone Wolf barrel. I need to do some more rested shooting, but what I have done doesn’t result in much better results than freehand. 6-8” groups at 25 yards are typical. Does a custom fitted barrel help much? When I install my barrel in the slide with no return spring, I have about .005” play forward and back and .010 side to side. Another barrel I have tha groups like shit has about twice that much.

In a rest removing as much human interference as possible it will make a difference but for the average shooter it is hard to say.  Only you can see if you would benefit from it.

What I can say though when I did my G23.4>G19 conversion I got a Brownells slide and Gen5 barrel because it was on sale for a good price.  I partially bought the barrel to see if the Markman barrel really was able to shoot tighter groups.  I had only tested it once with my neighbors G19.5 vs my G19.4.  While the difference was marginal a difference did exist when shooting prone resting on a range back.  Then swapping the barrels in the slides his Gen5 in my 4 and my 4 in his 5 the difference was still there.  When I got the slide and barrel together went to the range and used my G19.4 as the control and same ammo types in both pistols with 3 different types of ammo the Gen5 barrel shot consistently tighter groups in both pistols again.  We are talking very little difference in tighter groups but still tighter and repeatable.  Also each time we did this with two shooters, prone, resting on a bag and each group compared was same type of ammo.  Ammo used was Federal purple box 115gr FMJ, HST 147gr, and Winchester 147 Train(the PDX equivalent in FMJ ammo).

Here is the reality of it all ... do I notice a difference shooting off hand, shooting while moving, distance shooting (50-200m) or just general shooting steel ....... Nope I am still able to consistently hit my target repeatedly with the Gen4 and Gen5 barrel and only notice the accuracy difference when rest shooting.

I have a similar P80 build with RMR and Brownells slide.  I tried a couple different barrels and it wouldn't shoot better than about 4.5" at 25 yards.  Not horrible, but most of my Glocks shoot under 3" at the same distance.  I also noticed the barrel lock-up was fairly loose.  I ended up putting in a Bar-Sto semi-fit barrel.  It only needed a tiny bit of stoning on the front of the hood and then it dropped in.  It locks up much tighter now.  Groups shrunk by about half after that, and now I'm pretty happy with it.  YMMV of course.