Just checked: still the way I remember it.
ATK gains from weapon skills are linear.
Accuracy is a bit more complicated. Your "skill" for the accuracy check is the average of your weapon skill and your offense skill, so above 250 you're only getting half.
Also: level differences and DEX generally count for more. Your NPC target gets a "skill" value based on its level. If your skill value is lower, you get hosed. If your skill value is higher, your bonus is 1/5 of what the equivalent penalty would be for being lower by the same amount.
Quick example math: level 65, 305 effective DEX, offense = weapon skill but capped at 250, versus a level 50 target:
Code:
225: 66.2%
230: 67.4%
235: 68.7%
240: 69.9%
245: 71.2%
250: 72.4%
255: 72.9%
260: 73.7%
265: 74.2%
270: 74.9%
275: 75.4%
Thanks to level differences, we're always above par. Notice that 225 - 250 does count about twice as much as 250 - 275. (250 - 275 is a bit jumpy because the average of weapon skill and offense gets truncated [255 -> 252, 260 -> 255, effectively])
Now, all the same stats, but against a level 60 target:
Code:
225: 34.5%
230: 40.8%
235: 47%
240: 53.3%
245: 59.5%
250: 65.8%
255: 66.3%
260: 67%
265: 67.5%
270: 68.3%
275: 68.8%
Notice that below 250, you're getting owned.
I'm not sure what situation you were parsing 225 to 250 in, whether it was controlled for target level and dex and whether it focused on accuracy, but if it was in a below-par situation it's easy to see that it
would be a huge jump... but only because it's relieving a giant penalty.