Thanks guys. I might try fiddling with it again. It seemed as though when I put the new xx1 b-bolt in the gap (between the green lines) was greater 3-4mm than the gx as pictured.
To be perfectly honest, for the price, I think I'll probably jump ship to shimano now 12sp is out.... sick of all the sram issues with lower end models.
I am not sure if anyone else has replied to this and I missed it, but I just had the exact same problem with a brand new GX derailleur. I am running it as 11spd with an X1 shifter and Hope cassette. The first thing I noticed with the brand new GX is the B bolt is identical to the old X1, but being brand new there was zero slop in it, so it shifted fine from the get go. Since I had already purchased the X01 bolt, I figured I would install it anyway...
The X01 bolt made the derailleur shift so much worse than the stock GX and the reason for that is visible in the picture above (mine was actually much worse than pictured). The crush tube that fits around the bolt is ~3mm longer than the body of the derailleur. Since mine is new, there is no slop between the bolt and body, but the derailleur can wander in/out 3mm along the bolt and it throws everything out.
The fix is simple. File down the crush tube until you reduce that 3mm down to 0.000001mm or as close as you can get to zero without going too far and buggering it up so it binds. File-check, file-check etc.
Once the crush tube is the correct length for the GX body and the derailleur cannot slide in and out along the tube, the shifting is perfect!