xGUMMYx In a funny way, your question represents two separate dilemmas: GPU and Classification (for lack of a better term).
GPU: I believe it would have been completely appropriate of Asus to send you a 980 Ti, or even a GTX 1070. I'm somewhat shocked that they upgraded you to a GTX 1080. The GPU upgrade alone was in my opinion a classy thing for them to do.
Classification: Does Asus still sell Matrix cards for the Nvidia Pascal line of GPU's? I didn't see any listed on their site. I admit that I'm not well versed on the difference between the Matrix and the Strix lines. I honestly wonder if the reason they upgraded you to the GTX 1080 is because the Matrix was no longer available and they offered you an upgrade as a compensation.
Conclusion: If you want to battle Asus, and think you can convince them to upgrade you for free to a GTX 1080 Ti, then proceed. Personally I don't think they owe you anything additional in this transaction. No offense, it's just my feeling on the matter. Were it me in this situation, I would "ask" them if they would consider letting me pay the difference in order to upgrade to a 1080 Ti. If they responded "no" then I would either keep the 1080 or sell it and use the funds to buy the card that I wanted. That's what I would do.