You can subtract whatever Luiz cost from that bolstering because he just isn't very good. And Saliba too for the time being as he isn't actually in the squad as you mentioned.
You're going to tell me the bad player is Xhaka but I disagree, I think in the right set up he is highly effective just as Gareth Barry was. But this Arsenal team definitely isn't the correct set up. Guendouzi is fine, but not ready for week in week out play and Toreirra is supposedly good but I don't see it. Zat will probably tell me he just hasn't adjusted to the league again, but he's firmly out of favor and again I just don't see it. You've got some others like Ceballos and the young lads but none of them have made a huge impact.
The tl:dr there is that midfield is average and should have been more of a focus than a shot in the dark loan to be fixed.
I just think the transfer business was pretty lacking in terms of an overall plan. Lacazette and Aubameyang are already two players who play a similar role to each other. And that can work if a two striker system is employed but instead you've gone and spent big on a winger while also selling your only real player of any experience who is at home on the other wing. So now you have three forwards who don't really fit well into a system with each other, a midfield that doesn't really fit into a system and a defense that still isn't as strong as a "Big 6" side should have. Emery predictably is now having trouble figuring out his best team.
As far as the defence is concerned I think a bit of stop gap/forward thinking was going on and ended up muddled. He was a cheap panic buy when Leipzig refused to drop the price on Upamecano.
Xhaka does work within the system and the manager refuses to change it to suit him yet keeps picking him...go figure.
Iwobi is a good player but he needed a change,his confidence was low here and it wasn't working. Pepe was a better player but seems to be struggling to adapt.
Aubameyang and Lacazette actually work well together. As a partnership they've actually scored quite a few goals.
The winger system only works with one of each but that's not that big a deal means we can be flexible but we are not being flexible.
Torriera us our best to, you might not rate him but he is. Yet he keeps being moved further up the field in a stupid system
Watch him play for Uruguay in his best position and he's very good.
This is our best team
Saka Aubameyang/Lacazette Pepe
Guendouzi Ceballos
Torriera
Tierney Holding Sokratis Bellerin/Chambers
You could use injuries as an excuse but there are enough good players to fit more than one system .
This is on the manager.
In 20minutes when we are chasing the game we barely created a chance with 4 attacking players up front.
As organised as SU were when you have a passive system it's so easy for the oppo to get into 2 defensive banks whilst the ball is moving sideways all the time.
Probably a bit of a rant and a ramble but that performance plus the 2nd half against Watford suggests they don't have a clue what he wants and I'm not sure he does either.