TL,DR: You don't have to worry about any of those points.
The encryption is on a layer bellow the files, so from your games' point of view, nothing changed. It's almost like cloning your Samsung HDD to a Hitachi one, with exactly the same size, and putting it back.
Your games will still run with almost the same performance. You will incur on a very small increment on load times, because the back-end will have to decrypt the data before your game reads it, but the penalty will be almost negligible. Unless you have a program to measure the timings a couple of times, you will not notice it.
The same applies to your Dropbox files: they don't change. Only the low level representation of the file will be changed, but for your applications, they are still the same.
So encrypt your disk. Your files will be there, untouched, but remember the password. If you forget the password, your data is lost.