kiddiz, your story of your baby is very touching and no one on this thread would tell you that you should have persevered, blah, blah. These things happen as as has already been said, there are a multitude of reasons why people formula feed so no one can pre-suppose the position anyone was in when they made the decision to drop breastfeeding.
to address some of your enquiries: Formula manufacturers advertise heavily in to health professionals in journals so many midwives, hv's, doctors and scbu nurses have grown to believe that one formula is 'better' than others and when a professional 'chooses' a formula for your baby it endorses that brand to the parent. Most parents, my self included, will naturally go away thinking that this person knows more about looking after poorly babies than I do, so she must have made the best choice for me. This is one reason why there is a constant voice to stop the advertising of formula to health professionals. Instead, health profs should be recieving unbiased information from independent sources with regard to the ingredients and their function.
Likewise, parents should also be receiving unbiased information rather than advertising.
By the time i post, someone may have responded anyway but:
We do not know how exactly the individual person's IQ would differ if they were bf or ff, or how many gastric or respiratory illnesses or teh severity or duration of those illness would be depending on ff or bf.
What we can analyse is things like hospital admissions for babies with these illnesses, the number of visits to the doctors or A&E, the number and type of prescriptions written and the diagnoses for these illnesses.
By the sheer number and quality of the surveys in many countries, developed and underdeveloped, it has been shown that ff babies are ill more often, are ill for longer periods of times, have more hospitalisations for these illnesses, etc.
I have have a friend who bf her son for 11 months. He has had numerous ear infections since he was 6 mo old. My dd, also bf had her first ear infection when she was 3 yrs old. So when people give their individual anecdotes saying, oh, my baby was ff and has never been ill, really isn't here or there. It is by sheer weight of numbers of a population of say 10,000 babies, that we get a clearer picture of what happens when we formula feed, mix feed or breast feed. This is why many times feeding choices is talked about as a public health issue, which like all public health issues, touch everyone either directly or indirectly.