Having BTDT, unless you upgrade to a new amp w/ serious power... steer clear of 5.25" as their range is severely limited unless serious power behind them (40W isn't even close). You'll be extremely disappointed unless you pull out the stock "tuned ports" and make a baffle (links to DYI and template in archives here and on Boxster wiki) to fit 6.75 or 6x9" driver in there.
FYI - stock amp also has a crossover (no one that I've seen has been able to isolate the exact c/o point though). So, yes you want to go w/ either components or coaxial (and wire up only the woofer unless replacing stock amp).
Good luck
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