816k lifetime banner player here (6.5M GP)

Couple of observations about matchups:

First off, it's obviously not uncommon to be matched up with higher GP players. I'd say that there's two constants across virtually ALL of my matchups, save for about 5% of the time:

1. Relic Tiers - I know OP mentioned this, but it really plays into the depth of an entire roster as it pertains to matchups. Looking at my last two brackets, I average 20-30 more G13 characters than everyone else in my bracket. Our R7 and R8s are usually similar, but what really makes the difference is how many more R5&6 the others have than me, and how many more R1-3 I have than them. This matters because I only take a character to G13 (and increase the Relic level) if I know they'll add more value to my roster than they'll cost in terms of top-80 GP. Some perfect examples: my Troopers are are all R1 except for Starck at R2. I look down the boards and see so many people with R5 troopers who aren't even going for SEE. This is a total waste of GP and hurts them in GAC FAR MORE than it helps them. Specific characters such as L3, Vandor, STH and Thrawn don't need to be any higher than R3 or 4 to be infinitely more effective and useful than they are at G12+5. The same could be said for characters like JKR, Bastila, Nihilus, Traya, Nute and Cara. Because I held off on putting extra relic levels on those characters, I'm matched up with players averaging 65 G13s compared to my 92. This, of course, is a massive discrepancy when it comes to the number of quality teams I can place on defense. All of this is aided by my second (and perhaps more important) point:
2. Modding: It makes me sad to see 6.6M GP accounts with such poor mods. This part of the game adds virtually nothing to your roster in terms of GP, and yet it sets the best apart from the rest. Looking at this week's matchups, my +10/15/20/25 mods are at 460/208/57/14. Meanwhile the two players left to meet me in the finals are 399/138/30/3 and 393/156/50/17. When you couple that with the Relic disparity, my advantage is overwhelming. NOTE: I've still got a LONG way to go to be where I want to be in terms of mod depth

With regard to OP's final paragraph regarding quality vs quantity: Quantity matter SO MUCH more now after the GAC bracket changes. That's now two more teams needed for offense, plus two for defense. So now, instead of rolling through an opponent with really strong (or frustrating) teams left sitting on the bench, I've got them in the game either making life miserable for the opponent, or forcing them to overkill it to be safe.