Yep, johnl gave you the only possible answer. You've got to shoot them and find out for yourself. An inch difference will mean about a 5 pound difference in spine. Trying to predict whether they will shoot differently would be nothing but guesswork.

If I was guessing though, knowing Beman, I'd bet the 29.25" 400's are a bit weak in spine. I'd have to guess that 28" 400's would tune up and shoot better for you.