I agree with 90st. The speakers are already there, as everyone pointed out. Nissan isn't going to offer you much in the way of an upgrade, you get what you got. Even if they did, It would be at an outrageous cost, and not worth it.
See a local brick and mortar shop, and have them set you up with a new aftermarket head unit. Prices will vary depending on what model you choose, but Install shouldn't cost much more than $200 beyond your head unit with all the other bits required.
If this still does not satisfy you, you can look into upgrading your speakers down the road.
Another thought would be leave the stock system, and add a small amplifier to put more power into the stock speakers. You'd be surprised how much they can handle, and actually don't sound too bad doing it. I did this with mine when I first got it, while waiting for my new equipment to arrive. Sounded just fine.
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P99rs Head Unit--- Amps- 2 Pioneer Prs800d --- Zapco ST1000MX---Zapco ST-4XSQ---- SI BM mk5 Subs---SI TM65v2 Midbass--- SI M3 mids--- SI M25 xbl2 tweeters ----- 2016 MECA street class 3rd place at world finals
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