I've been shooting suppressed firearms since the mid to late '60s, my own since '81. While I've not specifically tested for increased accuracy, I have made several observations. Bottom line is with sonic rifle cartridges in thinner (sporter) weight barrels an accuracy increase is likely to the extent of 30 - 40% as mentioned. With heavier target/varmint weight barrels the increase in accuracy, if any, is generally less. If both are zeroed at say 200 yards w/o the suppressor the heavier barreled rifle will require less elevation adjustment to bring it back to zero when the suppressors are added. I believe this is simply because the weight of the suppressor does bend the barrel down a bit, the weight dampens the barrel harmonics and the added weight along with the suppressor design can reduce recoil. Of course, this refers to free floated barrels. Additionally, if you have a super accurate rifle that shoots well under moa then adding a suppressor will probably not improve accuracy or at least not to a noticeable or consistently provable amount.
As an example, I recently rebuilt my 30 cal suppressor [I am the manufacturer as I built it in '81] and have put it on my 308W pressure test rifle. The rifle has a 24" sporter weight barrel. It's had many test rounds through it and currently, w/o the suppressor, shoots 1 1/2 moa with 5 shot groups at 200 yards with a hunting load using the Hornady 165 SPBT over IMR4895 at 2700 fps and moa with the same load under a 168 MK. With the suppressor added it now shoots moa with the 165 gr load and sub moa with the 168 MK load.
Bedded barrels or rifle actions not conducive to much bedding improvements will generally not show any accuracy improvement. My standard ARs w/o floated barrels are an example, However, on Match M14s the addition of the Sionics Suppressor almost always improved accuracy. Match ARs with floated barrels also usually give better accuracy suppressed. Again, these examples are with sonic full power ammunition of 7.62 or 5.56 flavor.
With subsonic munitions and/or smaller pistol cartridges used in handguns or atypical rifles for those cartridges an increase in accuracy is not to be expected. My 22LRs, 22 Hornet, 223 Contender, M1 30 Carbine and M98 Mauser 45 ACP are examples. Of course, any suppressed subgun is still just a subgun and any accuracy improvement would be difficult to demonstrate.
Concealment is not cover.........