'Most IR multi carb setups are very drivable if properly sized......... .....'
You called it right there. Look at the engines and more specifically the size of the INDIVIDUAL cylinders. Then go look up everything you can on the term reversion. It can be controlled on SMALL cylinder sizes but not big. It CAN to a point but with huge work to get rid of the bad reversion side effects. The Pinto manifold in question here was developed around the time IR became the hot topic in gigonda V8 motor pro stock racing, the cylinders were too big and everything tried could not overcome it, not even Edelbrock, every manifold they made with true 100% IR and no plenum sharing made less power than when adding the plenum, which becomes absolutely necessary to absorb reversion pulses from intake pulses striking the back off closed intake valves, or backflowing from long intake timings at lower rpm. Bigger the cylinder the bigger the problem and why you won't see IR even now on carbed super inch engines unless the guy is losing. Head design can't change that although it can hurt or hinder. Your V12s mentioned are smaller cylinders..... ..........the more free flowing the head is the worse the problem.
Plenums are wonderful if you know how to manipulate them. They allow cylinders to share all carb bores, that damps reversion bigtime and allows for smaller carb since all the sharing is going on. When you go to true IR the carb has to be bigger to supply single cylinder per carb and that alone makes the reversion problem worse.
IR works great on smaller bike engines. Lighter weight of bike allows a motor built for high rpm get through the crap running at lower rpm a little easier but the big thing is CV type carb to not open carb all the way even though your right hand says 'yes I want it'. 90% of hotrod carbed bikes have them. Control to make up for reversion and too big carb to make power down low. All getting down to basic physics and functionality whether you want it or not and why engines today are generally smaller. Smaller engines are more 'air active' than bigger and can hit emissions easier for that. Air is easier to manipulate in smaller amounts than in larger, when bigger holes are used you get more turbulence, pressure pulse issues become unsolvable and you just get less air trapped in engine to make power overall size for size. Why you can get 3 hp/cu.in. out of bike motor and strain to get 2 out of car.
IR works on light rods again because of weight and the systems are made for looks and not max power, the throttle bores will always be too small for really good power. Take the effective throat size and compare to swept displacement of one cylinder and then do it on any bike, super difference there. Small bikes can have more carb on them than 1200 hp race engines....... ..........all in the physics of air, its' qualities, and how to handle them. Why you can have a stock bike run to 12000 rpm and no car on earth will (well, maybe a formula car, but look again, the cylinders are bike size!) either, air handling. Why you can't get 1300 hp out of a NA 302 under any circumstances but you can get 65 hp out of a say 17000 rpm Honda six cylinder 250 cc. engine (in 1966!). Same power output there size for size.
Check out the tach and the throttle response, can you say instant?
I got to play with its' big brother, the CBX, with 6-1 header the sound is awesome and pull your face off power. 11.50s dead stock 1/4 mi. with only a header back in 1979 and hazing the tire until 3rd gear. No other bike could do that at the time. First time I ever heard one was like wetting my pants.
By the way a stock 2.0 intake is as good as practically any you will find out there to pay big money for, and according to Dave Vizard. It straps the daylights out of almost anything including some sidedraft setups. I'd stay with it. It sure worked better on my 2.3 than the POS on there, and if building there again it would go right back on any 2.3 engine I made. I'm sloppy and don't like to pay big money and already have a low dog 600 cfm #1850 Holley that I will probably gut and seal the back barrels off of to make a 350 two barrel out of. I don't care what the engine looks like, just how it runs. 350 2 bbl. does pretty well on these Pintos.