It may not need to increase. I did some extensive analysis to help answer that question that I'll post up later.
For now, how much "travel to lands" would be a good target?
There seems to be a trade off between accuracy and pressure tolerance.
It probably also depends on other factors like bullet length and getting a certain amount of engraving done before the bullet exits the case mouth.
Yeah those are questions I can't answer since I've never loaded any of the heavier/long bullets, so I don't know just how much room we have with those bullets now and if it's to much or if it needs to be a touch longer, HTR would be about the only one that might know off hand if the .114 combined lead is good or if it would benefit from a change.
If there is still a bit of room from the ogive to the lands with the bullets seated to the optimum location in the case mouth for neck tension(one bullet diameter deep) then you may benefit from shorting the lead a little so you can jam the bullets if needed, it really depends on the bullets used and if you would like a jump to the lands on all of them or if you had rather be able to jam them in the lands.
IMO, which would be the best option depends on a read of which platform the military would prefer.
The Mk17 SCAR, M110/Mk11 sniper rifles, and the UK L129A1 squad marksman rifle, could be easily converted to the 2.50" round (needs only new barrel and mags). This would avoid the necessity to develop a mid-sized "AR12.5" platform.
However, the M16/M4 is by far the most widely used, and with which the troops have the greatest experience. This may give the edge to the 2.26" cartridge, even though it'd require what is essentially a completely new platform.
But, that's considering only rifles and carbines. For machine guns -- especially the M240 and MAG variants used by NATO countries -- the 2.50" round would (IMO) unquestionably be superior, both for performance and the ease of conversion.
The "big picture" (of rifle and machine gun) argues for the long cartridge over the short version.