If it was me, I'd leave it at the longer length. I have a 20" RRA varmint 1/8 and wish I had picked up the 24" barrel to squeeze out a few more fps. I think 20" would be the minimum to maximize the velocity/ weight trade.
To balance out the gun I got the lead buttstock weight for my A2 stock. It does a great job of making the gun not so front heavy. It used to balance on the front bag. With the weight in there now, it helps the gun to settle down when on the bench. And for $24, it is a cheap thing to try before you invest in chopping the barrel that you can't reverse without replacing.
As far as loads......
I have been happy with what I've found with the Hornady 68 grain HPBT Match. Win brass, cci 400, and Varget. Mag length got me under an inch. And 5 thousands off the lands got me under .5". I'd have to look in my log to see the powder weight but it was around 23-24 grains. They are not hot, but are spot on with my gun. The worst groups with this work up was 1.3" and average was .9". The load of choice ended up around .45" five round groups.