Just to clarify, is this a cat, or a silencer?
 

As per the pic, bought this 2-piece decat downpipe...and was under the impression that the secondary pipe was going to be a silenced section... but it looks very much like a cat to me....with o2 sensor as well.

Is it, I could be wrong?

http://img.photobucket.com/albums/v9...psxokrwyrl.jpg

looks like a cat to me

It is the cat, take a look inside if it is still in there. You should see the ceramic honeycomb

As above look inside and see if the cat has been gutted (insides poked out etc )

Look inside the heat shield as it might be a straight through pipe as I believe the Prodrive ones were but had oem heat shields fitted, which made it look like a cat.

If it's not a straight through pipe then 100% cat not resonated section.

Lets us know what you find.

This.

Also, even if it was decatted (it probably is, i'd wager), the o2 sensor has to go somewhere, as it can't just be left flapping around under the car :)

Good call, I'll get a torch on it later and have a look. I was told it came off a prodrive STI...so I'm hopeful it's just a straight through with heat shielding for effect. And yeah, I know that ref o2 sensor :)

The prodrive decat pipe with heatshield looks something like this. You can see that it is only a pipe

https://abload.de/img/img_20181103_184447x7zhci0.jpg