My audio upgrade for my FR-S:
https://www.etymotic.com/consumer/he...on/er20xs.html
With those in and stock system cranked up some it sounds decent enough, as good as it can with a much louder than stock exhaust system...
For me the FR-S is just not worth investing anything in the audio system beyond stock, way too much cabin noise (even stock) to be a good audio platform - a system delete makes more sense than an upgrade. The ER20XS allow you to play the system loud enough to get beyond the loud noise floor of the vehicle and provide an 'okay' listening experience. Damping the interior space and lowering the noise floor would be #1 priority before doing any kind of equipment upgrade on an FR-S. Even when not listening to music I have the ER20XS in for every drive... interior noise is too dang loud in it and it will effect your hearing which you will really notice on long trips.
My Nissan Frontier on the other hand, will be getting some audio upgrades on it... much better space for audio sound quality - stock system sounds absolutely great with high resolution FLAC playing from my tablet/with external DAC/amp feeding the aux port. Add in more sound damping and it'll compete with my home stereo setup.