The GPS antenna is IN the dash, you willnot see it unless you take the Nav display unit out.
Cell antennas are a big point of discussion in the indurstry.
If the phone is designed to take advantage of an external antenna, and you use a good one without too much cable it will help.
There are enough bad cell phones, bad cell phone holders (or other external antenna atachments) and bad cell phone abtennas out there to make any particular set up either better or worse than just using it with the phone antenna.
So in the end, buy good stuff, and you are ahead of the game.