And if the compression is on the lower side, the cat may be slowly falling apart and pieces of it getting sucked into the engine via reversion, scratching the bores up.
Thought this might be worth its own thread, since its slightly OT in the original.
I wonder how likely this is.
Car in the OP is a Nissan Altima. Dunno what model but 2.5L engine was mentioned. This might be a picture of the upper exhaust manifold and catalyst (the squid-head thing). http://i.ebayimg.com/images/i/131110816600-0-1/s-l1000.jpg
I assume this is a potential issue when both valves are open.
The manifold seems to be for a four stroke engine, so IF say both valves are open for 10% of the downstroke, it could potentially suck-back about 60 cc's. Though the catalyst is fairly close to the exhaust ports, the intervening volume looks to be more than that. If a failing catalyst was likely to wreck an engine that'd be a fairly widespread concern and I'd have expected to have heard of it, but I might have missed it.
I don't have a catalyst but have re-invented the potential problem due to my practice of making oversized aluminium exhaust gaskets and allowing the exhaust gases to "burn them to size". The melted aluminium in the exhaust is probably a good thing, but in the cylinder, not so much.
I also sometimes put cement and aluminium in the downpipe, but that's further away from the exhaust ports and so probably less of a threat.