Kershaw Designs 14.4v DeWalt conversion is simply a kit to make the DeWalt conversion a little simpler. There is a dual motor version of it, too.
The 14.4v DeWalt motor is a pretty commonly used "upgrade" to the 775. More power, speed, and torque than the 775, but still uses the stock EVX2 and remains waterproof. Definitely not the all out speed of a brushless, but its more than stock. It's also pretty cheap, since the motors are usually $35-$40. The brushes are replaceable, too, so ongoing maintenance is not a big deal.
As far as CorrosionX is concerned...many people are using it successfully to "waterproof" their ESCs in a Brushless setup. Just understand, if you are considering this option, that CorrosionX is not a waterproofing product. It coats the surface with a highly water repellant coating. This can be enough to protect the ESC for 90% of the water exposure it might see, but it isn't a 100% foolproof solution.
I'm not suggesting that CorrosionX is a bad idea or that you shouldn't do it. I'm simply suggesting that if you don't have the budget to go out and replace your ESC if it gets fried due to water damage...consider this option very carefully.