I finally hit 375 in Fishing yesterday - woot! Getting your fishing up is no small task, let me tell ya. Thank goodness they are making some much needed changes in the next patch. It won't make the grind significantly faster, but it should cut down the frustration level.
First I can confirm that the number of successful casts does not ramp up significantly from 300-375. In my first test I was getting a skill point every 9-10 catches. At the end I was getting a skill point every 10-11 catches. It went up slightly, but not enough to really be a problem. I guess Blizz decided that a 10% skill-up rate was enough aggravation, lol.
I also want to take this opportunity to remind everyone that fishing is more lucrative than it used to be. Certain fish provide sought after food buffs such as the Sporefish and Bluefin. I was able to sell a stack of Sporefish recently for 20g, that's pretty good money for fish. I regularly get crates that contain ore/engineering parts and other various crafting mats. Fishing also happens to be one of the easier ways to get motes of water, which are almost as expensive as air and fire on my server. Fishing can be a nice way to break from the normal farming grind and do something productive, yet relaxing. So c'mon -- give it a try!