First off - congrats on your new Porsche
Having been down this road.... I'd say your components are correct, but I'd change the order as follows:
1. Install the rear speaker system
2. Change the elements in the dash-speakers to better ones
3. door-speakers...... (whether you do or do not have existing ones - this is a must do) If you install a sub w/out doors, you'll be missing a whole lot of low and lower end mids. Dash (at 4") will really only be upper mids and highs and a sub is well just a sub.
4. Install a Sub somewhere - wait until you have evaluated steps 1-3 b/c no matter where you put it, it's going to take up space that is already at a premium for taller drivers/passengers. Plus, with the right set of door speakers you may not find it all that necessary unless you really like thumpin' bass
YMMV
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