Pioneer speakers on oem radio, what to expect?

Hi there

My Jetta has the std VW Gama head unit with the std paper cone speakers still installed. It sounds quite nice. Will installing Pioneer 6.5inch single cone speakers in the oem's place make a noticeble difference or am I wasting my money? I would like better bass responce from the system without going the woofer and amp way, on a bit of a tight budget you know.

Any Sugestions

I replaced my stock speakers with some infiniti speakers and the treble immedietly sounded better you could hear more than with the stock speakers but the bass got worse. In fact when there was alot of bass in the song everything sounded bad. But the speakers I used are high wattage speakers with a cross over so I dont thnk the stock deck could power them very well. http://www.cardomain.com/item/INFKAPPA607CS

Then I replaced the stock deck with a JVC deck and it sounded 1000x better than the stock setup.

Not a bad economical choice. Crutchfield throws in the speaker adapters so plug n play.

http://www.crutchfield.com/S-97KD1IXpyXC/cgi-bin/ProdView.asp?s=0&cc=01&g=52000&tab=review&i=023GTX652&pg=2&sortBy=3

The pioneers I am looking at is a single cone unit rated at 30w nom and 160w max. So I think they wont be too heavy for the head unit as it puts out 20w X 4

No such thing as "too heavy" for the head unit. As long as they are 4 ohm speakers. That's the load the speaker puts on the unit.
In my car (98 Beetle) I have two Vifa home speakers (4 ohm) in the doors. Two Alpine co-ax speakers in the rear side panels with a cross over so that they are mid-range and high. (because you can't get bass out of the flimsy side panels.) I then have a powered Bazooka sub in the back and spliced off of the rear speaker wires to just have line level input for it.
The head unit powers the doors (mid-bass, mid, and treble) and the sub just adds low bass. It sounds AWSOME!

One thing to REALLY look at is the stiffness of the speaker cone motion. Many high wattage car speakers have very stiff suspensions in order to handle the power. Stiff suspensions make the bass suck. That is the reason I went with Vifa home speaker 4 ohm. in the doors. They have a really pliable suspension and a strong magnet to control the cone. The bass from those is really nice.