Lara I just tried to post a long response to that carseatinfo link which has alarmed you so much and lost it. Will try to recall what I said
Basically I really don't think that you should worry on the basis of what it says in isolation.
Firstly I have not come across that site before and it seems to be run by a company called Kidex ltd who also seem to have links to Baby2000, a retail store, so not sure what independence or scientific credence it has.
Secondly it quotes a BBC report from 2006 based on a study in New Zealand of 9 infants (surely not a meaningful statistical sample)which doesn't differentiate between prolonged use in car, general use and those left in outdoor clothing indoors. Even so the article starts "Babies should not be left asleep in car seats unchecked because there is a risk they will stop breathing, warn experts"
Lastly the substantiating evidence linked to are US or Japanese based and the site fails to cite any similar UK or European research.
Some infant car seats are more upright in structure than others but the angle it lies at much depends upon the fit in the car, shape of seat and height/angle of seat belt by which it is restrained. You may well be able to lie it securely flatter than it seems when stood on the floor. Viaggio iirc has a flatter base which may not fit the profile of some cars as snuggly as others. Best advice is to go to a retailer who will try a variety of different seats in your car to determine best fit and most comfortable back angle.
Please don't fret about long term damage on the basis of this report it is far from proven imho that it causes ADHD etc. Even childseatinfo concludes "If you are currently using a conventional infant carrier find out from your supplier if you can recline it back and in the first few months keep your journeys as short as possible. Under NO CIRCUMSTANCES stop using it."
hth