Let's try to go to the basics - do you agree that, provided using the same (and I mean the same) technology , the bigger light capture area has better signal, thus theoretically better IQ?"Same Technology", I am assuming you mean same pixel density. The picture quality (noise) per pixel is identical regardless of the acyual sensor size. The FF will have more pixel than the APS-H and hence better picture quality. Hydyn1971 is half right about the comparision of APS-C ans APS-H with the same pixel density. besides a widen angle with the same lens, The APS-H will also have a higher resolution and hence better opicture quality.
There are quite many variables currently, we need to use ceteris paribus principle ie fix some variables to understand each other.
Let's define couple of situations:
1) Let's suppose the same density PLUS the same technology used as a different technology with the same density can give quite a lot of variations in image quality:
1a) a camera with a cropped sensor
1b) a FF camera
2) Let the density vary meaning both, the crop camera and the FF will have N megapixels, again using the same technology.:
2a) a camera with a cropped sensor
2b) a FF camera
Now some thoughts:
3. For the first two cameras, 1a and 1b, yes, no problems to understand the benefits of a larger sensor IQ wise as it just records more data on a bigger surface (some characteristics may however degrade, for example fps when being constrained by the data processing capability ie MPs per second).
4. For the second case, let's suppose that for a given print at least 300ppi is available, I claim that ISO-wise the camera 2b is much better on paper.
The fear for APS-H advocates seems to be:
5. by being under data processing constraint, the crop camera, having the same number of megapixels, would end up with higher density thus better "reach"
6. FF is more expensive
In my opinion, at certain level of density in combination with high ISO (meaning fine detail is being lost), the benefit is questionable (look at the compact small sensor cameras, the density for SLR-s is going towards that). This is why I think it is time to change to FF in order to provide competitive
I also, let's be dramatic, hate lenses acting as being handicapped. It is a big deal for example 24-105 not acting like one. After all, the world is not composed of tele lenses only.