What's the faster controller for my M4 SSD?
The Intel one. I have a Marvel controller for two SATA III ports, I found out they don't perform very well (330MB/s vs. 550MB/s)
Should I hook up my DVD drive to a 6GBs port?
No. The DVD drive is slow as hell, and nothing's going to change that.
What is TRIM, exactly?
It prevents the OS from writing over areas marked "unused" in the file system. When the OS deletes a file, it doesn't actually erase it off the drive. It just marks the space it took up as "unused". If the OS were to write to the first available block of space in the drive, it would wear out the SSD faster in those areas while others remain untouched.
It's a form of wear leveling.
If I wanted to change controllers, what would be the procedure?
Just switch where the drive connects to.