Juuuule - you are right that in a western context, it can be hard to 'spot the difference' between bf and ff kids, and this is partly because ff babies grow adequately so you can't point to the skinny, runty ones and say 'aha!'
It's also hard to judge when you have a small sample - in a western context, again, you need hundreds in a study for the differences to show up, as you need to control for socio-economic status, pre-term birth, inherited illness and whatever else, so you know you are comparing feeding method alone.
Fortunately, we do have a lot of evidence, from western countries, that across a wide range of health factors, not breastfeeding increases the risks.
You are never going to get a randomised controlled trial for this, obviously, so the best studies look at lots and lots of babies, to overcome this difficulty.
In the UK, there are a number of good cohort studies. We also have studies which only look at babies in the developed world, as there are different factors in places where the water isn;'t clean, for instance.
Here are some links
pediatrics.aappublications.org/cgi/content/abstract/119/4/e837?etoc is the Millenium cohort study in the UK, which shows the difference between bf babies and ff babies in terms or diarrhoea and hospitalisation.
www.ahrq.gov/clinic/tp/brfouttp.htm is an overview of studies done in the developed world.
adc.bmj.com/cgi/content/abstract/91/1/39?ct more serious illness in this case coeliac disease in ff babies
adc.bmj.com/cgi/content/abstract/92/6/483 another good cohort study, this time looking at asthma, coeliac disease and obesity.
I hope these help answer your question.