I believe my Boxster came with the m490 Hifi system package and I added the rear shelf speakers. I had a nice set up in my previous convertible with focals and honestly I've never complained about the sound with the top down in the Boxster. I recently started using a FiiO E18 portable DAC for my Android phone (also doubles as a phone charger when not in use with just a push of a button) and the sound is pretty good.
Would I see that big an improvement over the stock speakers?
http://taingheviet.com/upload/sanpham/FiiO-E18-Kunlun-FiiO-E18-Kunlun-2.jpg
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M490 - Comes with 6 speakers, amplifier (6 x 25 watts), and multi-channel loudspeaker system with base reflex boxes in doors."