I just read an article in the May 2005 issue of "Rifle" magazine that covered your exact question. In the article by Chub Eastman, he was able to test two 300 WSM
barrels at the Nosler ballistic labs. His goal was to be able to load the .300 WSM to an OAL of 3.000 inches. One barrel had a SAAMI chamber, the other was chambered with the same reamer except for a straight .250 inch leade (similar to what you did to your barrel?).
Quote from the article:
"Starting OAL was 2.820 inches and was increased .060 inch at a time until an OAL of 3.000 inches was reached. Three shot groups were fired at each AOL. In theory, as the bullet was seated farther out, the subsequent drop in pressure would require an increase in powder to maintain loads at factory pressure. The first surprise that proved this theory was questionable came when the second series of shots were fired when AOL was set to 2.880 inches. No increase or decrease in powder was needed to obtain near the same pressure or velocity using the barrel with the modofied chamber. Surprise again, when we found the same results at 2.940 inches OAL and at a full 3.000 inches AOL."
Although there was virtually no change in pressure or velocity with the modified chamber and seating depth changes, the SAAMI chamber acted more like expected with higher pressure and velocity the nearer to the lands the bullets were loaded, so anyway, this may be similar to your situation. As for your velocity and pressure signs, again, your barrel modification probably is why you see lower velocities and pressure than expected, (although they are in the ballpark).
I personally don't like to play around max pressures if I don't have to, hoping to find the best accuracy and barrel life a few percents below max.
I hope this helps. Just remember, there are so many variables in reloading and individual rifles it's hard to pinpoint an exact explaination for why you get certain results.