From teh testing I have done, what I have witnessed from actual velocity testing with the same load in different barrel lengths is that a round of this size will generally gain around 20 to 22 fps per inch of barrel added after 24" out to around 28". From there on out you will gain about 18 fps per inch.
In a round like the 257 STW and my 257 Allen Magnum which have case capacities in the 85 gr and 100 gr range on the very small .257" bore, you will see around 35 to 40 fps gained goign from a 26" barrel out to 30".
It simply has to do with the expansion ratio of the round and the length of barrel you are using. A round like the 257 AM has a very low expansion ratio so it will gain more velocity per inch of barrel then say a 257 Wby with its higher expansion ratio.
Your Warbird has a pretty low expansion ratio but not exceptionally low for a 30 cal round. For thsi reason this round will not get a huge velocity gain per inch of barrel.
The big 338 mags are even worse in this aspect. They will generally only gain 20 fps or so at the most per inch of added barrel.
Heavier bullets will gain more velocity in long barrels then light bullets will, simply because they use slower burning powder.
I certainly will not question anything that one of the Jarrets say about their rifles but I will say that by todays standards the 300 Jarret is a relatively small 30 cal magnum. Basically identical to the 300 Wby with its 85 to 90 gr capacity compared to the RUM with its 100 to gr capacity, the Warbird with its 105 gr and the 30-378 with its 120 gr capacity.
SO of all these it will gain the least amount of velocity per barrel inch and it is far better suited to a 25" barrel then either of the others.
These numbers are not a given by any means, more like an average. Some barrels will produce slightly more, others slightly less per inch.
If your looking to gain 50 fps per inch of barrel you will be disappointed I am afraid.
The way I look at it, the 300 RUM and the Warbird, in real world rifles using real world bullets are damn near ballistic twins when each are loaded to the same chamber pressure in same length barrels. They also will gain about the same velocity per inch of increased barrel length.
I assure you that if you took a 300 Jarret and chambered it in a 38" barrel, you would most assurely gain anywhere from 75 to 100 fps over the 25" barrel length so, yes, they will keep gaining velocity, even out to 36" or so of barrel length.
Jarret rifles are built for extreme accuracy, well all custom rifles
are built for this but the best way to get extreme accuracy is with the stiffest barrel you can use. That translates into either a shorter barrel or a heavier contoured barrel. The Jarret rifles are designed to be relatively light weight and as such they use shorter barrels to get keep their barrels stiff.
Again, accuracy is far more important then extreme velocity.