Live and learn right here all for free... LOl.
Yes you would loose speed sensitive
And yes a higher power "quality" amp is going to get you more volume.
And since upgrading speakers wasnt on my list of to-do's I didn't delve into it.. but check on the 2 ohm 4 ohm speaker thing.. lol.
Also if I'm not mistaken the rear subs are dual voiice coils.. something else to consider..
Adding the amplified bass cannon with a line tap to rca was my first "upgrade"... then adding the new HU..then breaking out to four channel input to the amp and redoing the bass cannon feed to the HU dedicated rca plugs was the extent of my sound upgrade, so anything past that is just what I remember while searching through threads..