First off, I want to clarify that the "shopping slumming" phrase itself is just a bit of fun and very much tongue-in-cheek. Hope no-one gets offended by it. :)
This might lead to some head shakes, but more recently I've adopted a loose policy of only paying full price for items that are made for me. Beyond that, I've veered away from the borderline absurd "bells and whistles" of the top-tier OTR makers. I no longer see the utility or charm of a completely handmade garment if it doesn't fit properly. Buying a Kiton shirt because it is completely hand-finished and then taking it to a tailor who will make alterations on a machine seems completely useless.
While I don't currently have the budget (or the time) to go completely bespoke/MTM on all of my work attire, I find myself buying cheaper pieces (or pieces that can be had for a higher discount) as I save for the items that I really need. I've also narrowed my list down of stuff that I need/should be looking for (as stated in an earlier post here).
We're on a fairly similar page. I keep a rough mental list of what I'm after; the annual "sartorial reflections/plans" (or whatever it's called) threads we have on SF at New Year's is actually really great for this!
However I don't mind paying retail for RTW items whose fit is within an acceptable (to me) deviation from bespoke, regardless of whether that's low-end or high-end RTW. The number of such items/brands is really small though, so the net effect is similar to your principle I suspect. The exception to my RTW rule is where the high-end is expensive enough to get somewhat close to bespoke pricing anyway, in which case I won't buy it unless the styling is absolutely spot-on to what I want. Personally, where this tactic has the most effect on my spending is in blocking any MTM orders. MTM never quite gets me what I want and is expensive enough relative to equivalently fitting RTW that that I've now stopped ordering it, in favour of much rarer bespoke orders instead.
Where we're on the exact same page is an increasing level of diffidence towards hand-detailing in clothes (whether in RTW or otherwise) if it doesn't fit right. I just don't get the same emotional/aesthetic reaction to handwork that others do. The whole artisanal connection thing passes me by. TBH, I have the same reaction to high-end mechanical watches. I have a relatively cheap Longines with - I assume - an off-the-shelf movement. The only thing reason I even have one mechanical watch is that I find a sweeping second hand pretty. My other watch is a quartz (Seamaster).
I also really agree with thinking about clothes purchases in an opportunity cost way; it makes for very easy budgeting and delaying gratification.
I normally avoid synthetics too. Certainly on anything that actually makes contact with me. But a small percentage isn't the end of the world in a jacket if the garment is spot-on otherwise and it's very cheap. As long as the vast majority of the fabric is natural, my feeling is that it breathes/reacts closely enough to real thing to be tolerable. But yeah, everything else (including minimal price) has to be the case for me to cave. Apart from the Gap jacket I got today, I think the only other (non-technical) item where I applied that principle is a Zara jacket.