I don't think it really matters whether Leeds is the biggest financial centre outside London or not as that is not the main driving force behind LBA expansion. Elements of it could definitely help any negotiation, but it is not the key to it. In fact, the Leeds City Region development council always quotes its air and rail links with London as being central to further growth.
With regards to the North West, the airport and its worldwide links most definitely aid the area, as well as other, more spurious reasons such as Manchester United FC, city wide light rail, regeneration of the city centre following IRA bombing etc. But I can't see such growth being possible without a reasonable means of getting into and out of the place e.g. Manchester Airport. Let's remember the rail link to the capital was poor until recently, certainly inferior to that of Leeds.
LBA can't expand like that - the apron is already full overnight in summer - but if it does the right things (improving access being my main point) there is no reason that it can't successfully house a small long haul operation, whether Leeds is the biggest financial centre outside London or not.