I sure have to agree with Rocky on this account. In fact, I had a Remington 742 in 243 Win and a Win Model 70 in 243. With identical loads the 742 shot faster over my chrono. Maybe the Remington just had a "faster" barrel than the Win.
So will a gas auto shoot slower? ,, maybe.. but sure not enough to make a bit of difference downrange. Mine actually shot faster,, go figure??