Not sure what car it is that Simon is talking about but I suspect Japanese and someone has come up with the same implausible theory that affects early versions of the Toyota 1ZZ-FE engine fitted in the MR2 Roadster. The theory is that the cat innards start to break up and somehow make their way back into the engine so wear the bores.
Talking about the Elgrands.
I didn't think it very plausible myself at first either but now I know for fact that it does happen and have suffered it myself.
I reckon the way dust from blocked exhaust makes it's way back into the engine (where it acts as a grinding paste) is due to the excess backpressure and the exhaust valve remaining open past TDC on the exhaust stroke into the beginning of the intake stroke, at which point the inlet valve will have already opened a bit dumping some of the cylinder pressure* into the intake plenum... Now there's a situation where the exhaust is under far more pressure than the cylinder and the exhaust valve is open.
*At this point in the cycle the cylinder would usually be at low pressure, the exhaust valve has been open during the entire upstroke of the piston, pressure should be low enough for intake charge to start flowing in through the intake valve as soon as it opens. Except now because of the blockage the exhaust itself is still under pressure, the cylinder cannot be below exhaust pressure so cylinder pressure is at high exhaust pressure, so when the inlet valve does open cylinder pressure flows the wrong way into the intake... which lowers the cylinder pressure, which (since both valves are open) allows exhaust to flow at least into the cylinder and some will even flow into the intake plenum.
Like Bri says, the above effect will be worse on a 2 cylinder bank engine than on a 1 cylinder bank engine... the 1 bank engine might not run with a really severe exhaust blockage but the 2 bank will because one bank's exhaust isn't blocked. And some of that exhaust that's being dumped back into the plenum will be a high dose of EGR for the other cylinder bank on a 2 bank engine but not a high enough dose to vastly limit that bank's power. Or put another way - on a single cylinder bank engine it isn't only the exhaust restriction that lowers power output but also any backpressure can see the intake getting a lot of EGR further reducing it's power, but on a 2 bank engine the EGR couldn't lessen the amount of fresh air getting to the bank without the exhaust restriction by much more than maybe 50%.