It is impossble to predict with any accuracy. Even guns with equal barrel lengths often will be 100 fps or more different. I've seen guns with fast 20" barrels shoot faster than guns with slow 22" barrels. It could be as little as 50-75 fps slower or as much as 150 fps slower.
To be honest you'll be fine. At worst it will still be about the same speed as a 308 from a 22-24" barrrel. They kill stuff pretty well.