Sorry in advance for the novel...
A year or two ago I got the chance to conduct some pressure and velocity testing with a legit pressure test barrel and it kind of opened my eyes as far as "seeing pressure". At the point you see pressure signs in a well-made custom action, you're probably in the 68,000-75,000psi range. SAAMI max for this cartridge is 65,000. So somewhere in between when you see a flat primer, or an ejector swipe or whatever it is, and the actual SAAMI max is 3,000-10,000psi unaccounted for. Max published loads tend to follow that SAAMI max avg. pressure pretty closely (this is where SAAMI chambers, the same brass, bullet, etc... plays a part-- deviation from what was tested causes deviation in pressures).
Anyway, there are a couple reasons I don't typically exceed those loads. The first is that things work better. I almost never get a stuck case, sticky bolt, etc.. Cases briskly extract and eject without issue. Cases last 10-30 firings-- in my last 6.5 barrel I averaged 22 firings on Hornady cases. Once you stretch the case out from too much juice, the primer pockets don't "heal" over time, they're toast. So my price per shot for brass goes to almost nothing. And finally there's the barrel. If you run the stress/strain equations for a thick-wall pressure vessel with a typical barrel contour for the first 4-5" of bore @ 60,000psi internal pressure you'll see that it's on the edge of yielding (plastic deformation) in the ID of the bore. Granted, a firing event isn't the same as static internal pressure loading, but the moral of the story is that the hotter you load, the higher the pressure, the more work is done to the bore. The wider the bore expands each shot before contracting back to original dimensions, the more work is done to the material, the sooner it will work harden and develop cracks (the last sentence purely theory on my part, but it does seem intuitive and appears to hold true if you look at guys loads vs. round count to burnout).
Long story short you get more case life, more barrel life, easier running, safer system, at the expense of 50-100fps. Run the numbers out to 2500yd and see how much 100fps at the muzzle helps you in wind variability. If I want that speed back (in this case I do), I get it through barrel length. If you read a few pages back here you'll see at least one poster that smoked primer pockets in 3-4 firings without seeing pressure. You see the same thing in 6.5 SAUM and 6.5 PRC cases-- Hornady makes their stuff to handle SAAMI pressure and not much more. You can get away with more with some other brands but it doesn't necessarily mean you're "safe" or that you're doing your equipment any favors. YMMV, to each his own, etc...
So with that said, I think shortening up the throat would increase pressure-- as a general rule that's what happens. I don't know if it would increase velocity, but it might, slightly.
I was around 3.715-3.720" COAL for 225's to touch the lands. Ran them at 3.700" COAL. They go 2940 fps with 74.6gr of RL26. Doing a linear interpolation for bullet weight vs. velocity, the 5gr of difference in bullet weight would make up about half of the ~35 fps change from 225 to 230 both with 74.6gr of powder (2905 vs. 2940)... So yeah, jumping an extra .080" probably has something to do with it, about 15-20 fps worth maybe.
This is really subject to individual barrels, but I did not find a satisfactory grouping load over 75.0gr. 75.0 was similar to the above picture, but SD was around 8 or 9 IIRC. Figure your ES is usually about 3x your SD so 1.5 up and 1.5 down-- past 1500 MV spreads start producing really significant POI changes.