The only way you will actually get more mv out of a longer throat in a given setup is to allow more run to the rifling. Anything else, as Mikecr has said, is no real surprise at all to me.
I always chuckle when people try to load a 220-230gr pill to the rifling in a 30roy (toss any other heavy pill and long throated round in if you wish) and wonder why they had pressure issues before they topped out with book data. Let her have a run at the rifling and see the difference. Set both barrels' ammo to the same oal and see what happens. Even then, it shouldn't be more than a couple of % better mv at the expense of a lot of powder.
Freebore is a great thing to flatten a pressure curve as you mention. That's how Roy Weatherby made his money. Sadly, it can have very poor effects on accuracy depending on how the throat is set up as far as diameter.
This system works great for most chambering and most bullets but you do reach a certain point in velocity and in bullet when jumping a gap to the lands will result in very poor results.
Most of my wildcats are prime examples of this. Running a really long, heavy bullet into a fast twist barrel will wring it out like a wet towel and as a result the jacket/core bond will weaken. What happens is that on these heavy, very long bullets, the front half of the bullet will engage the lands and begin to rotate but because the bullet is so long and heavy, the rear of the bullet will resist this rotational force and it takes it toll on the jacket/core bond and then bad things can happen as far as accuracy goes. Again, with MOST chamberings that produce less then 3200 fps with these heavy long bullets, generally you do not see problems. But when you take the same bullet and slam it into the lands and then rev it up to 3400-3500-3600 fps and at best, you have very poor accuracy results, at worse, you have total bullet failure when the bullet leaves the muzzle because of the stresses from slamming the bullet into the lands and then running the velocity up and things come apart.
The only cure for this is a heavier jacket to handle these high stress loads.
In full custom rifles, that have a throat diameter that matches the bullet diameter very closely, I have not seen much accuracy drop off with freebore until you reach the point that the stresses imposed on the bullets cause the jacket/core separation mentioned previously.
One thing I do not like about long freebore is that you can see more flame erosion in the throat as there is more room for the hot gases to slip past the bullet in the throat. This will erode a throat faster and the longer the throat, the more distance a bullet has to travel before it engaged the rifling. As such, accuracy can drop off faster with a long throat then with a short throat.
Most match bullets recommend seating to or just barely off the lands for best accuracy. In fact Berger bullets usually shoot the best seated into the lands. For a hunting rifle I do not recommend that but that is what they recommend for best accuracy potential no matter if its an 80 gr 224 cal bullet or a 300 gr 338 cal bullet.
