White Sound Defense has conducted some testing with regard to how their FOSSA series of flash suppressors effect the accuracy of the firearm that they are mounted on. They posted images that came out of the test showing that the FOSSA-762 seems to have little effect, if any, on the particular precision bolt-action rifle used for testing.
It has been my experience with AR-15s that, as long as the muzzle device is properly torqued (a good reason to use shims instead of crush washers), the effects on accuracy are marginal and hard to measure. This test conducted by White Sound Defense may not be scientific but it shows anecdotally, that your accurate rifle will remain an accurate rifle with a FOSSA attached to it.
If you are looking for a very effective flash suppressor that can also reduce muzzle rise, the FOSSA series of muzzle devices are a great choice.