If you already have tweeters, you don't want coax. They would lead to either having 2 tweeters running parallel, at reduced power, or the one in the coax being deactivated if there is a crossover. Depending on where exactly the factory system is lacking, you can either upgrade the woofers or the tweeters or both.
The Ford systems are eq'd (frequency corrected) to the speakers and the cabin. I don't know what Nissan does - if it is the same with them, even nominally better speakers can give a worse result (if the frequency response is corrected where it is not necessary).
You can try (and err) yourself, or find a trustworthy shop (or mate) who knows what they are doing.
With the last upgrade I did myself, only changing the tweeters helped a lot for clarity and reduced distortion at higher levels.
If you are looking at new woofers, the available depth is a major criterion in selection the correct replacement.
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