when in manual mode and S# for 08+, it will shift at redline. if the gear indicator changes gears it doesnt mean its now in that gear. it's probably to inform you when the tcu prepares for the upoming shift. atleast in mine, i have an mt map flashed on my car.
no it's not dangerous equalizing engine speed(at very small, to no loads) to car speed while shifting results in the least amount of friction. engine braking is fine(also called dynamic braking). and ecu and tcu will not allow you to overrev or switch to a lower gear causing overrev.
yes, except unbolt the downpipe from the midpipe connection, not the shorty section, it's easier. and you'll need to unplug the rear o2 sensor, but that is plugged in right above the pipe
I hear that subaru recommends pressing the accelerator a little while manual downshifting to speed up the shift, like rev matching. The more you can equalize the engine speed with the speed of the car in the gear you want to go into will make a smooth downshift. Some downshifts are hard even in a healthy 5eat