Catalytic Converter Dead ( think)

I have a 1996 Disco 3.9 V8i XS Auto with an LPG system. I am pretty sure i have somehow killed the catalytic converters as there's no oomph in the engine now. Kickdown doesn't kick, goes up hills at about 15 mph!
I've little experience with mechanics (yet) but as i intend to make this into my latest offroader for next year i reckon a tucked away exhaust would be good. Do i really need the full exhaust system with the cat pods integrated? Would it be cheaper/better if i just effed the front bit of the exhaust and the cat off and had a straight through S/S system? Would this have a bearing on the emissions for the MOT? Sorry for my lack of knowledge but I would appreciate your input on this if anyone has a minute. Many thanks.

Emissions for the MOT won't come into it if you present your disco for mot running on gas.
An iffy cat would not give the lack of kickdown, have you checked the cable adjustment ? have you checked plugs, leads, rotor and cap?

do the cats rattle when you hit them? found bits of a customers cats in his rear silencer blocking it off last week!!! could always try slackening the downpipes at the manifolds and going on a noisy test drive, if the cats are blocked the exhaust can breathe through the gaps between manifold and downpipe, noisy as hell but works!