The Nosler 40 is somewhere in between the Hornady 40 and the Sierra 39, you might try a box of them to see how your rifle reacts to the slightly shorter bullet. The BC is close enough not to matter much, so you won't give up much if anything, and you might gain quite a bit in accuracy.
I do a poll on that forum about once a year, and so far, the 39 Sierra wins every time, by about 50% compared to ALL other bullets combined.
Neither of my .204s really likes heavier bullets, but I can get "useable" results with the Sierra 39 and a stiff charge of H-4895 (which is very similar to R-15, come to think of it), nothing spectacular, but about 5/8" or thereabouts.
About the only thing I use my .204s for, are prairie dogs, so I just use the 32s and plan my shots carefully. Anything will kill a PD, they aren't tough. I don't mind the shorter ranges, I can't shoot well enough for those 500 yard shots anyhow.
Go to
www.204ruger.com and check out the stickies there. On one page, I made a photo of the three longer bullets, Sierra, Nosler and Hornady, the pic is posted there, and you can see that the Hornady has a much longer boattail than the other two, enough to change the bearing area of the bullet, so it pretty likely that switching to one of the other bullets should help your rifle handle them better. I wouldn't get stuck on one bullet, especially if that bullet isn't working out for you.
I hope this helps a little. I hail to Glenn Asher over there, if you register. I'd say I'm a moderator, but we don't have to moderate those guys, they are all pretty well-behaved.