There are some minor loads which do not develop the same ft/sec velocity per grain of powder as higher loads. For instance in a 50 cal, a load like 25 grains. Likewise once you reach an upper point, the velocity increase starts to drop off This point in a 50 cal is around 110 grains of powder. While ft/sec continues to climb, it does so at a decreasing rate, until the increased velocity is negligible. However, muzzle velocity isn't everything. all patched round balls lose velocity as they travel down range, due to air resistance. Not even this is at a standard rate. As the patched round balls exceed the speed of sound, their rate of deceleration (loss of velocity as they travel down range) seems to increase. So a ball launched at 1,000 ft per sec, will lose a certain amount of velocity by the time it reaches 100 yards. A patched round ball launched at 2,000 ft per second actually loses a higher percentage of it's velocity by the time it reaches 100 yds. Some say this may be due to air piling up in front of the ball and causing increased drag.
This becomes important when some shooters increase powder to flatten their trajectory or increase their hunting range. According to the Lyman Book's ballistic tables, (which some have said are abnormal figures) it can take as much as 50% more powder just to increase range in ft/lbs by 15 or 25 yds. (this of course depends on barrel length, powder and caliber)
Also according to the Lyman ballistic figures, in 54 and 58 caliber guns, the extra powder for extended range idea seems to be born out by their tables to an extent. I'm not sure why the larger calibers tend to keep increasing velocity per grain better than the 50 and under calibers. Part of it is probably due to the fact that the larger caliber balls carry more of their initial muzzle energy down range than smaller caliber balls. (a 36 cal ball loses about 65% of it's intial muzzle energy at 100 yds. A 45 caliber loses about 55%, a 50 caliber loses about 50% and a 75 caliber loses only about 25% of it's intitial ME at 100 yds. These are pretty rough figures but the Lyman ballistics tables show the trend of energy retention by ball weight at 100 yds.)