As far as velocity gains are concerned, you may get 15 to 25 feet/second more velocity per inch of barrel. I actually like 24 inch barrels quite a bit and wouldn't be put off by the minimal velocity loss to be expected from them compared to a 28 inch barrel.
The Encore is short enough to begin with that even with a 28 inch barrel it isn't any longer than a lot of other rifles. That said, I don't see a big advantage to 28 inch over 24 or 26 inch barrels. In fact, a common question coming up is about whether or not to shorten the 28 inch fluted barrels. In my opinion, a shortened barrel with the flutes extending clear out to the muzzle looks dumb.
I think the market is also saying something about 28 inch fluted barrels since many are coming on the market that are lightly used, in excellent condition for very good prices. The reason I have 4 of them now is because they shoot very well and were available in stainless steel at prices I couldn't pass up (I know, I'm a barrel junky).