your headunit isn't the bottleneck here. the stock speakers/tweeters are pretty mediocre at best so even getting just a sub and removing the low frequencies from your speakers will improve the sound quite a bit. Either way, a Bluetooth connection is going to compress the quality of your audio hence why plugging into AUX will sound noticeably better.
..and yes, as long as you get the proper wiring harness you can retain the backup camera + steering wheel controls to the HU but be warned you do lose the stock microphone if that matters to you..
good read..
http://www.ft86club.com/forums/showthread.php?t=111866