My guess is the base Z1 model will perform roughly on par the Steam Deck on GPU-limited scenarios (newer games) but a lot better on CPU-limited ones (emulation, older games at high framerates). Asus is probably going to charge for the base Z1 512GB model about as much as Valve charges for their 512GB Steam Deck, around 650€, their case being that it has a better screen and it's not limited to Steam games.
The Z1 Extreme model is anyone's guess, at this point. If Asus charges >1000€ I think it's a failed product and it will sell about as much as the AYA Neo, GPD Win or AOKZoe. If it's 800€ or less then many people will be willing to pay for it for sure.