Personally i would do as Embee suggests, check the handbook to see if the car features speed related automatic volume adjustment. And if so whether this has been switched off or is adjustable.
Also check how many speakers the car had originally. It may be more than just the two door speakers . Check they are all working, and whether left/right and front/rear balance can be adjusted. Its possible that a higher spec model has more speakers , than your car . (perhaps dashtop speakers etc) . Fitting extra speakers from a higher spec Jazz might be an economic way to improve performance.
I am no great expert on Hi fidelity sound. I may be entirely wrong in saying that whilst improved speaker performance ,better bass, better positioning etc may bring benefits , ultimately its just a better quality at louder volumes

. You already have scope to 'pump up the volume'.
Its would be easy to spend £200 + with limited benefits if you get it wrong. I'd rather put it in a piggy bank and be £200 closer to my next car.
