Hm. I think this does depend on the PoD. I often see the "Brown bounce" being attributed to Brown being seen as a decisive leader and his being seen as more of a straight talker than Blair. As events unfolded, Brown was definitely seen as less decisive, and I think that very much hurt him. And then of course the Financial Crisis hit which rather flipped the political apple cart.
Had Brown opted to call an election in 2007 because he was feeling more decisive, I can see him winning with a reduced majority.
If he called an election in 2007 but was not feeling decisive, I think winning with a threadbare majority or having a hung Parliament are more likely, since voters will pick up on Brown's discomfort and hesitancy.
Either way, I think this has a huge impact. If Labour gets a majority, the response to the financial crisis is dealt with MUCH differently. Brown is often credited for organizing the international efforts to blunt the crisis, and it's notable that international coordination became less coordinated and effective after Brown lost in 2010. Now, some loss of coordination may have been inevitable. But a longer period of Labour government could well mean a very different economic trajectory both for Britain and the world.
If it's a hung parliament, I think there's good odds the Conservatives form the next government, even with less seats or a lower popular vote than Labour. At the very least, I suspect many in Labour would want to out Brown as leader after calling a snap election and loosing all of their majority. And as others have pointed out, Tory internal polling said they'd likely emerge with more seats in a hung parliament. Such a government would also mean a very different reaction to the financial crisis an apparent Tory bungling of the economy (since the crash would hit the UK under their watch) which would have interesting political effects.
I do wonder what Sir Menzies Campbell would have done as far as coalition forming, had such an election resulted in a hung parliament.
fasquardon