This argument that he's not good enough for Arsenal, and as we want to be where Arsenal are, therefore he can't be good enough for us is a little simplistic.
Different teams have different strengths and weaknesses depending on how they want to play and who is available to them. How many teams would have started Victor Moses in the last two years yet he has a Championship medal. Gordon Strachen's career was supposed to be over when he left Man United yet he won a Championship with Leeds.
We need pace and all the goal threat we can muster and Walcott is an obvious upgrade on what we have.