I'm not sure on the nav question, having not ordered with Nav... but possibly if you get the ML audio system it'd use the center speaker (siince the ML system has one).
As to handling, just drive one. I test drove a TL-S, and it was nice, but the IS350 was better in pretty much every regard... better interior, better features, better power, and being RWD, much better handling without the annoying torque steer. The TL-S does a nice job for being FWD, but you just can't beat physics... steering with the drive wheels just isn't a good design.