While looking in to some pre-detonation issues (at high speeds under load, engine loud rattle/misfire and wont go above 7k rpm) discovered that my CAT is gone!! I mean, completely gone...just an empty pipe.

I have a CEL light too but the only error code is "intermittent MAF sensor" which is odd as I would have thought driving round with no CAT would have thrown some kind of error too. I replaced the MAF about 4 weeks ago and cleared the code, but it has come back. Is it possible the wrong reading from the CAT lambda sensor could give a phantom MAF error code?

Anyway, new CAT installed. But why did this happen?

I've done some searching here and I think next job would be some new coils (am I right?). I am still getting the same mis-fire at high speed/high revs despite the new CAT which is also pointing at ignition problem.

Anything else to check/replace to prevent the CAT melting again?

Any advice on this is much appreciated.

 

It is possible the maf error caused a lean condition (excessively hot exhaust) that lead to the cat failure. I would think that an ignition problem would create unburned fuel, therefor a rich condition. Too much unburned fuel in the exhaust could also lead to a cat failure.
Along with changing the ignition parts, you may want to check and clean the maf.