The unit I have is NOT Rockmars, however, my headunit is Chinese too and as I have heard, it is better than Rockmars in quality. I have used two navigation headunits, and I never faced any issue with performance of GPS. Since, I have not personally used Rockmars, so I cannot really comment on the quality of that particular brand.
Hooking up a cell phone with car LCD is something which I don't want to endorse. It just does not seem right to create a "jugaar" in a Rs. 2 million vehicle.
Apart from aesthetics, there are reasons behind NOT USING CELL PHONE for GPS in car. First of all, unless you use offline maps, lot of data movement would be required. Unless you have an unlimited package, so much data transfer is going to cost a hell lot of money. Secondly, we don't have 3G services offered by any cell phone company. As a result, there will be substantially long time before the image on the phone is refreshed. Thirdly, the maps on cell phones are not meant for car navigation, even if the company claims. I have Nokia N8 and it has options for car navigation, but the screen is too small. You cant fiddle with large number of touch screen or qwerty buttons while driving. It can be extremely dangerous.
Now ... my cup of coffee has arrived ... I will come back with more reasons after I refresh my mind.