I don't have an opinion in this, just an interest.
What are we trying to do with the ideas - cheaper, lighter, flexible, wider range of temperature (cooler) ?
So I've not wasted any $ Nikwax proofing my TH, but I just stood in my bedroom and tried wearing my TH under my Litespeed and tried my Quito and then just let heat build and in an unscientific way feel the insulation. I'd say the Quito was slightly warmer due to a more head-encompassing hood than the TH+Litespeed, so when I unzipped a little they became comparable.
The TH (not proofed) +Litespeed (proofed in stuffsac) weighs 630g, my Quito (proofed) weighs 500g. Also the TH+Litespeed roles up larger than the Quito, I'd say comparable to ratio in weight.
The flexibility you may gain on wearing insulation & windproof independently is offset by only one form of venting - the front zip, but a TH+FA has the lined up zips (at even more weight).
It's also made me realise I can see the inside of the liner, there's some used in the small hand-warmer on the Quito, it feels like soft smooth velvet (soft version of a snooker table covering?) but its not fluffy, there aren't spikes of fabric which move like the fluffy side of the TH.Similarly the smooth sides very different, the liner is slippy smooth the TH rougher. The liner is thiner. They're both bobbling
Holding to lips andblowing through, the liner is slightly less wind-resistant, say comparable to the differnce in fabric
I suspect what someone may do is very specific to what they already own and what they add to it, rather than blank-sheet make a specific decision. e.g if someone had got Pull-on in a sale stretching it into a waterproof direction.
The TH+Litespeed is more urban-friendly I'd say.