I think this transfer step will bring Arsenal to the level of success next season, although on the other hand the dominance of Man City, Liverpool, Man United and Chelsea will make their steps difficult.
The EPL is very competitive and the top clubs are making very good signings to ensure that they remain competitive and challenge for trophies, the transfer window closes in a month or so, thus there is still time for more signings, so right now it isn't possible to know the team that is going to make the most out of the transfer window, but for now i am impressed with how well Arsenal have conducted their transfer business.
Having said that, City are the favorites to win the EPL again next season, but i am pretty sure that it is going to be a tougher campaign for them, teams like Chelsea and Liverpool that were bad last season will be stronger in this new season, so it will be more difficult overall.