I never found much need for a dedicated upper in .22LR only when you can use you regular AR 1/9 with the 60 grain SSS bullets and get excellent results! I have 2 different conversion units, a Ceiner and a Surplus M261 when I use the Ceiner in my 1/9 twist with the SSS my point of impact at 100 yards is pretty damn close, however with the lighter bullets is does not perform as well. Now on the other hand when I use the light 36 or 40 grain bullets either Sub or Super Sonic in my Colt 1/12 upper it performs pretty well again with the POA being close enough to be considered "Center Mass" at 100's Also when I use the SSS bullets with my Suppressor they are unbievebly quite not to mention nasty on ground hogs at about 75 yards. Keep in mind though that if using Sub Sonic rounds in a suppressed hand gun, they tend to not like the heavy SSS bullet and instead prefer the 36 to 40 grain Sub Sonics. This is especialy true in the Walther P22 with the short Barrel installed. In short it just doesn't have the time to stabilze in the barrel before entering the suppressor. Of course this is assuming you are using a suppressed fiream.
As for me personaly though I do not think I would invest the money to buy a dedicated upper when either the M261 or the Ciener will perform as well as they do! Just seems like a waste of money that I could buy a complete different firearm of some type, possibly even another silencer! :> (Since I got my first suppressor recently I have fallen in love with silenced firearms and have since bought 2 more, now I am just waiting on my Form 4s to get back, should be about another month or 2 but Hey who's counting day's!!!! :}
Ok back to the twist rate question though, I suppose if you just wanted to invest in a dedicated upper and wanted to fire standard .22LR High Velocity Ammo then I would suggest going with a twist closer to the Ruger 10/22 (1/16???) but if you just want efective and cheap shooting I would stick with my standard AR .223 upper and the SSS rounds. Even without the silencer they are still very accurate and I like that the POI is so close without any sight adjustments at all!Again the 1/9 twist likes the 60 grain SSS ammo suppressed or not and the 1/12 likes the standard 40 grain Remington High Velocity and have provided me with hours of shooting fun for a fraction of the cost spent on ammo. This is not to mention the suprise you can give the fellow on the bench next to you when you quietly open your AR and swap the bolt and suddenly that cute little "pop pop" goes "BANG BANG"!
But Hey! To each his own, this is my .02¢ worth! Anyway, hope it helps you a bit RemMax
P.S. if using a standard .223 upper to fire .22LR's all day all it takes to clean your gas system out is to fire about 5 rounds of good quality .223 ammo though it at the end of the day then clean it as you normaly do.
Also as for the Ceiner v.s. the M261 .22LR adapter, both perform equally as well. RemMax