His numbers will be (much) better than Aguero's, but will we as a team score more goals than we did with Aguero? It doesn't depend only on Haaland, of course, but the all important question is not about his individual numbers but about City's numbers.
As things stand, Arsenal won't have a player with 15 PL goals at the end of the season. What's more, Haaland alone may outscore their top 3 PL goalscorers this season. But it won't mean much if Arsenal win the title. Football is a team (first) game.
I think Haaland is a phenomenal striker and he can help us more in Europe than Aguero and Jesus, hopefully already this season. Aguero was fantastic in the PL, but not in the CL.