That kind of confirms my point! The Eicher rear pads had an anti-squeal shim attached to the back whereas the equivalent Pagids has nothing but a (badly) painted finish. Pagid's 'solution' to the squeal issue is to angle the face of the friction material instead? How convenient - makes the pads cheaper for them to manufacture (on two counts) while claiming a design improvement.
Chamfers are just as valid an anti-squeal measure as shims. As for cost, as I said in my previous post, the chamfers will add cost. I've never seen Pagid's chamfered pads, but normal practice is to mould the pads without the chamfer & then machine the chamfers, so you've got the cost of extra plant & tooling plus extra waste to dispose of. . . possibly still cheaper than shims, though. You say that the backs of the Pagid pads were "badly painted". Was it a sort of rippled effect? If so it may well have been rubber paint, another anti-squeal measure.
Different manufacturers have different palliative measures to reduce squeal. What works with one friction material may not work with another; what works with one caliper may not work with another; what works on one vehicle application may not work with another . . . it's a minefield!
PPS. Get this - just did a bit of research and discovered that the manufacturing company that market their products under the Eicher brand (Brakes India Ltd.) also manufacture products under licence for Bosch and (surprise) TRW. They supply OEM parts to, among others, FIAT, Ford and Mercedes in certain territories. They are also very well respected in the commercial vehicle sector.
Here's a little bit of information you may find surprising: the friction materials a manufacturer supplies as OE may well be different to its aftermarket products.
As I said, I am not familiar enough with Eicher products to criticise them; notwithstanding that, my money goes to the names I know & trust.
Eicher discs should be fine though, as one lump of machined cast iron is likely to be pretty similar to another.
Up to a point, yes. Cast iron may be a low-tech product, but it exists in a number of grades, with a wide range of tensile strengths. Certain properties of the cast iron can affect coefficient of friction & hence brake performance. Research in the late 1960s/early 1970s found that the presence of certain trace elements in the material, which are not controlled during the casting process, could affect friction levels to such an extent that using two discs from different melts could cause the brakes to pull to one side. Dimensional accuracy is also important.