I was in the same boat as you and I went into the dealer and sat in two vehicles, one's with and one without using a CD of some Commodores Music with lots of bass.. the difference was ESPECIALLY noticeable at louder settings, but it wasn't so much a loudness difference as it was a clarity.

The speakers on the Premium system are far better, with substantially larger magnets, and there are more of them. The amp is also of a superior clarity. Lastly, you get the CD changer, which frankly I didn't really care for.

Get it if it doesn't mean you have to skimp on something else you absolutely must have. As a "Stock" system, the original system isn't bad at all however.