Every thing on your computer uses drivers to operate. You installed in IDE mode. There were no AHCI drivers installed as you were not using it. When you did the reg edit, you told windows to load AHCI drivers, so now it will run in AHCI mode. It was not Windows fault. Windows didn't install raid drivers either. If you want to use a different mode, you have to load the drivers for it.
Powered esata is a esata port and a USB 2.0 port combined. When you hook up your drive it will run off of the esata or the USB 2.0 bus. If you hook up a USB 3.0 cable to it, it is the same as if you plug a USB cable into a USB 2.0 port. All you're going to get is USB 2.0 speed. If you want to really increase your speed, get a powered esata drive dock or enclosure. The reason the port is powered is because it gets it's power from the USB port. In truth, it was never really intended to be a USB port, but that was the way to make it powered.The USB port will provide enough power to run a 2.5" hard drive, but rarely a 3.5" drive. esata is faster than USB 3.0 in practice. It does not have the overhead of USB.
The answer was here AHCI : Enable in Windows 7 / Vista