Cold air next to hot exhaust valves is what I've always thought burnt valves....how it got there is what the member with burnt valves will need to tell us. If too free-flowing exhaust, then the swirling exhaust air could've pulled outside air back up to exhaust ports. Leaking exhaust flange possibly could've done it, but the pipes woulda told on him(pop on deceleration).
90bowtie, yes, your thread got derailed, hahaha. I'll look at that cat but I will tell you now that the saying "you get what you pay for" applies alot to automotive parts. Especially with cats, some use cheaper internal metals(instead of pure platinum; an alloy of it), or sometimes the internals are just rolls of screen(aka dummy cats).