Okay, let's just take a step back, and observe what Romney brings to the ticket, both in positives and negatives. Broadly speaking, it looks something like this:
Strengths:
1. Moderate Republican can strengthen McCain's maverick message and appeal to moderates.
2. Economic background might give him some credibility on that subject.
3. Appeals to Mormons, possibly compensating for the republican's failures in that arena.
Weaknesses:
1. Has supported positions at odds with much of the republican party.
2. Mormonism is a problem with evangelicals who form a major republican constituency.
3. Disliked by large segments of the Republican party establishment to some degree.
4. Has little resonance with the Republican party base, or really much of a core of support in the party at all.
So, what does this mean for the election:
1. Mormons will vote even more strongly republican, but given that they do so already there would not be much change from this.
2. McCain would be hurt among the right wing, religious conservatives in particular. Expect far more to stay home or vote third party than IOTL.
3. McCain can act more convincingly like a maverick, but arguing that he represents change will be nearly impossible.
In short, McCain may do a little bit better out west, but probably not enough to swing any states. Meanwhile, his own party will grow disillusioned and, with a weak turnout, lose more convincingly in battleground states. Expect the Republican party to swing far to the right come 2012 (Palin, Jindal, Huckabee, somebody like that).
Honestly, I think that McCain did about as well as you could realisitclly have a republican manage. There just isn't any way to swing things decisivly in his favour without losing somewhere else, unless the entire dynamics of the race change. About the only VP choice that could make a difference would probably be a Pawlenty figure, and I wonder if even that would be enough.