I don't know, 225lbs is a lot, but it still shouldn't sag that much. I've put more weight than than in the back of mine, and it barely made a difference in the static height (this was even before I went to higher rate springs). I had my coil overs set on lowest setting and even with 300+ lbs of stuff, it wasn't as low as yours.
The comments your installer is making gives me the impression he doesn't know what he's doing in terms of coil over install. If he set the preload on the shock correctly, there is no way for a spring to pop out. They are separate, but you are supposed to adjust the spring height, then jack up the spring mount and adjust the length of your shock until you get to 5mm preload. So basically the shock is now 5mm shorter than the spring so the shock would top out before the spring ever got loose. If he doesn't know to do that, then I'd be questioning things. And on the flip side, if he adjusted the shock too short, then it wouldn't matter where you set your spring height, the shock will just preload the spring and your rear will stay low. Long story short, check your preload.
And springs don't settle, if they did eventually all of our cars would be on the ground. If the install was done correctly and control arm bushings adjusted to new ride height, the car won't get any lower. It can't. Cars are just higher when you jack them up and put it back down, as soon as you drive it, it's at it's normal height. If you jack up your car right now and put it back down, it will be higher even if you didn't touch the suspension. As soon as the car moves, it's at normal height.
---------- Post added at 07:01 AM ---------- Previous post was at 06:48 AM ----------
That system seems like it's more built for events/shows and such, like a rolling party. Doesn't seem like he could drive with the 2 boxes just sitting like that, doesn't look like the hatch would close. Plus it doesn't make sense to aim full range speakers at the hatch. But even without the extra speakers, the sub/amp box is huge.