Someone may need to correct me on this. The adjustment from the LOC is like the signal strength or volume control, and the adjustment on the amp fine tunes the signal to remove noise and adjust frequency and gain. The bass control knob does the same job whether it comes from the LOC or the amp.
Your amp probably has adjustments as well, so what I did was leave the radio loosely mounted to get a rough adjustment on the LOC, and then made finer adjustments with the amp, then button everything up. I ran the ground to the spare tire well with a self tap screw, it's worked fine so far.
The shop I used wouldn't use 2.5" pipe because they said it couldn't make the bend around the spare tire well without crushing. A "better" shop may not have this problem, but then you have a 90 degree bend at the tips, which is another issue to tackle. We ended up using a tip like this and...
They didn't want to put remote start on a manual. My wifes '18 EX has remote start, my '20 Sport touring 6MT does not, purely because it's a manual. You have to get an aftermarket remote start and find someone willing to install it on manuals.
It's not about "need", it's about the look. If they made it a dual exhaust and shortened it I think people would like it better. The coupe and hatchback are the same length exhausts, the sedans are a couple inches longer.