Zampa:
You said: "The one coat paints, especially the oil based glosses and satins reley soley on going on thicker..."
Actually, I don't think oil based paints go on much thicker, but the will dry to a thicker film. I don't know why, but oil based paints can be mixed to a higher solids content than emulsion paints, and it's only the solids that remain behind to form a film as the paint dries; all the glycerine added when tinting, all the thinner and the solvents all evaporate. (and the film thickness of the paint becomes smaller as these liquids evaporate from the paint) So, if you're buying a pre-tinted oil based paint (like an alkyd floor paint or a white alkyd primer), much of the reason why it will have better hide is because the stuff inside the can has a higher solids content, and so less of it evaporates and you end up with a thicker dry film over the substrate.
"Primarilly, common extenders here have only been used to give the paint body without affecting the colour."
Extender pigments also perform three other important roles:
A) they are used to lower the gloss of the paint. Without extender pigments, all paints would dry to a high gloss film. They do make colored pigments in different "fine-ness", and a flat white tint base would be expected to contain a coarser grind of the white pigment titanium dioxide than a satin or gloss white tint base, but all of the pigments in the paint tinting machine will be finely ground so as not to lower the gloss of any high gloss paints that are being tinted.
B) emulsion paints form films through a process called "coalescence". Emulsion paints consist of a mixture of clear plastic blobs (called PVA or acrylic resins) and solid particles called "pigments" (including clear and white extender pigments and colored pigments) suspended in a solution of water and a low volatility water soluble solvent called a "coalescing solvent". When the paint is applied, as the water evaporates from the wet film, the plastic blobs find themselves surrounded by the coalescing solvent at ever increasing concentration. The coalescing solvent "dissolves" (kinda) the clear plastic resin blobs so that they become soft and easy to distort out of shape. Capillary pressure and surface tension then work together to make each soft blob pull on all it's neighbors so that any spaces between the blobs are eliminated and the paint becomes a continuous solid film with the pigments suspended inside it much like raisins in raisin bread. The coalescing solvents then evaporate out of the fresh paint film over the course of the next few days (giving off that "freshly painted smell"), and as that happens the plastic blobs harden back up to their original hardness.
The tension that can develope inside the paint film as the soft plastic blobs are all pulling on one another can be quite considerable, and can even cause a poorly adhering underlying paint to let go of the substrate as the overlying emulsion pulls itself together during coalescence. Extender pigments lessen this tension by occupying much of the paint film's volume and breaking up the tension that would otherwise develop in the paint film. (It should be noted that ONLY emulsion paints will develop tension in them as they cure. Oil based paints don't do this at all. So, if you're in a position where you have to paint over a paint that's not adhering well itself, it's safer to use an oil based paint than an emulsion.)
C) Extender pigments, in conjunction with the type of binder used, are also what determines the "scrubbability" of a paint, which is how well it stands up to hard scrubbing without losing it's gloss. Basically, the principle reason why flatter paints are softer than higher gloss paints is because the most commonly used extender pigments are softer than the plastic binder the paint is made from. So, the more chaulk or talc you add to plexiglass (Perspex), the softer the overall film becomes.
To get good scrubability, you need to use a HARD extender pigment (like pulverized silica sand) so that you get a harder film instead of a softer one with the addition of extender pigment. BUT, in order to derive the most benefit out of that hardness, the hard particles need to be well supported by a plastic binder that dries to a strong hard film. Thus, to get good scrubbabiliity in a paint, not only do you need to use a more expensive HARD extender pigment, you also need to use a more expensive binder that dries to a stronger harder film to properly support those hard extender pigment from being rubbed off in hard scrubbing. And, this is one of the reasons why better quality paints cost more. You can scrub a better quality paint harder to get out a stubborn mark without roughening it's surface.
Maybe print this off and show it to the Dulux rep you'll be meeting to confirm that I'm not making all this stuff up.
Are they harder to apply for an talentless amateur like myself 
