What are the issues with the screen? Some issues are surprisingly easy to fix.
When I first bought my 4.8is 3 months ago, the stock Nav screen was missing the colour blue completely. I was quoted $300 to have this fixed, however I fixed it myself in about 30minutes at no cost.
If you are having issues with sound quality, unfortunately installing an aftermarket head unit won't do much, as you will still be using the stock amplifier (DSP).
Have you tried playing around with some of the DSP settings?