Theres nothing but improvement when you change the old 10cm speakers at front... Even the cheapest and crappiest aftermarket speakers you can buy at "gipsys selection co. ltd" are a lot better than what the old and tired speakers are. Even if I bought the most expensive and best 2-way 10cm speakers to the front they wont do much good when the tweeter element is releasing sounds at your feet. Exaggeration? Not to me
What comes to 6x9. Theres also perfectly good holes for 4x6 speakers and you can easily make some adapter plates out of MDF or plywood to mount a set of 6x9 in there. Plus thats more of hidden setup than putting them on the parcel shelf where every lowly scum can see what to steal next.