Pretty much any system would benefit from a speaker upgrade. It's not just the power you're radio is putting out, but the components used in factory speakers. Factory speakers are usually made from paper or cardboard while aftermarket speakers use rubber, graphite or other high quality composites. Ultimately I'd still say figure out a way to get an aftermarket deck or amp in there, but some people don't want to go that far. It's really pretty easy to do. I don't know if there's a difference in wattage between NAV vs. no NAV, but I doubt it.