Your information is correct. There is alot of opinion out there from people who've worked on old pushrod V8s and/or other obsolete stuff.
Modern catalysts, particularly ones with metalic substrates don't represent much of a restriction at all. Also, modern production cars , usually dues to emissions and because they use variable cam phasing and also as they are predominantly 4 valves per cylinder (needding less duration) have minimal overlaps compared to the days of old so are effected less by backpressure then older cars were.
I could post flow bench figures, but to me they are meaningless unless they are going to be used as input into an engine simulation package or are used for comparison purposes comparing one type of catalysts against another- you can't compare them to port flow data . Also the data changes depending upon the pressure drop you use. There is also often quite alot of flow bench to flow bench varition , and alot depends on how the flow bench is set up.
The other point of note is that a flow bench can only subject the catalyts or subject component to a steady state flow, NOT a pulsing flow as you get in a engine.
With this in mind I'm now posting data for V6 engine, of cycle avaraged data, while the engine was running for 180 hrs on a high speed dyno at 6000 rpm (with an average engine mass flow rate of 870 Kg/hr). This engine represents a stage 4 emissions compliante engine with a particularly challenging package - so is perhaps worst case for today cars.
It achived an average of 69 kPA per bank at a gas temperature of 940 deg C, measured at the catalsyst cone. Another tapping post mid muffler after the banks had come together showed about 16 kPA at 819 deg C under those same conditions.
To quantify these figures, this modern state of the art vehicle with it's backpressure redued to just about 39 kPA -non restrictive single brick monolithic catalysts and no other modifications went from peak torque and peak power figures of 163.1 kW and 278 Nm to 168.8 kW and 287 Nm. clearly this engine is MORE effected at the peak torque region then peak power due to its higher overlap at those engine speeds (more advanced inlet cam timing).
The largest benefit that could be realised if a close coupled modern catalysts is removed is to replace it with tubular exhaust manifolds to get a "tuning" benefit. Also re mapping so that the car no longer runs Stoichimetric (14.7 parts of air to one part of fuel) but runs lean at part load operation will reap ENORMOUS fuel economy benefits.
