BHX does have spare capacity - but only if you want to fly in during the quiet periods - 02:00-07:00, 11:00-12:00, 20:00-21:30.
Assuming the runway can take an extra 9m passengers today, the terminal isn't big enough to cope, the immigration halls are definitely not big enough and the staffing levels aren't there (UKBA, Servisair etc). Arriving passengers are frequently queuing up the stairs and down the international pier during the busy periods at lunchtime and midnight.
9m extra passengers today? No, get the staffing levels right and you may have an extra 1-2m next year. Anything above that and you'll need to do some building work.....