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[Diamond Black Geezer]

Just wondering - and if so, how?

I see three options
-replace the entire front section of the exhaust with a straight-through custom made one
-remove the exhaust section, and physically 'poke'/chop out the internals, leaving a void, effectively an empty catalitic converter/venturi chamber
-just chop out a section of the exhaust with the cat, then replace with a short section of straight pipe.

My own pros/cons list is...

pros
ooh arr more powerrrr bwwwum bwuummmm etc in a Clarkson voice (possibly gains of 0.01 to 0.1 bhp  )
saves noisy rattling exhaust
cost - probably far less than replacing the cat section with new/good used (which itself may start failing very soon too)

cons
may have detrimental affect on engine running, because every engine is designed with a certain amount of back pressure in mind. remove some of that back pressure and you are actually making the engine run worse
time & aggro vs replacing with another cat section.

For legal purposes I must state that I do not have a rattly old cat, which is only getting worse, and do not intend to replace it with a straight-through by however means, and then simply replace the cat shortly before Mot time. Definitely not. 

[Diamond Black Geezer]

Also take a seat for the various feline-themed jokes that inevitably appear whenever anyone mentions 'cats' on a car forum 

[Marks DTM Calib]

Cons - MOT failure.

[Diamond Black Geezer]

Hence the last 'disclaimer' line. 

Just looking into the idea at present. Just weighing up the options, seeing if anyone's done it... 

[zirk]

Tried it years ago on a BM 325 LPG, as was given some strait thru's for free and thought the MOT Man would be happy, as long as it was presented on LPG.

Outcome was bloody noise'y and seemed to make bugger all difference on power. 

[Diamond Black Geezer]

Cheers. That's 2 in the 'no' camp so far 

[Mr Gav]

I did it on My Senator and just put them back in for the MOT, ran well with the chip in too, it was worth the four gaskets a year, although access was a lot better than the Omega.

[Diamond Black Geezer]

And 2 to 1, now 

Thanks Mr Gav.

[TheBoy]

TheDaddy had a decatted 3.0, ran awful. After putting back to how it should be, it was back to how it should be....   ....sideways everywhere. Just don't tell him about the last bit

[GastronomicKleptomaniac]

I'm a bit light in the head, but wont it baffle the ECU if theres no reduction in exhaust nasties?

[MrRon]

I remembered a friend mentioning a catalytic converter could be replaced with an X-PIPE.
Could be worth a try? 

http://www.ebay.co.uk/gds/How-to-Remove-a-Catalytic-Converter-/10000000205355240/g.html

[Diamond Black Geezer]

Re: TB's post, I can see, even encourage how decatting certain cars (perfect example perhaps a Carlton/Senny 3.0, who's engine was never designed to have cats in the first place, so grab a 1991 model and remove them and you've got some extra bhp for free) Mr Gav's statement, therefore I can see as being completely true.
but the Omegas V6 being that bit more finely tuned, I'd guess there'd be some very advanced fine fettling involved (with ignition timing, ECU? or have I got that wrong?) in order to 'correct' for the slight reduction in back pressure. If ten people post saying they've decatted with good results, then I'd be happy to agree, equally, however, as with your evidence, I can just as easily appreciate how it would run poorer.

Re: Gastro's post, being a 2.5 I don't have an 02 sensor aft of the cat, so I don't think the engine 'knows' what emissions are being chucked out.

Are we into the same territory as a Fuel Pressure Regulators? 'A bigger FPR means more power' more fuel = more power. Which of course, it doesnt work like that.