Hi all, hope someone can help me with my problem!

I have a 91 model 190E (1800cc Manual transmission, with catalytic
convertor)
Driving home along a the motorway yesterday, realised the car would not go
over 80mph in top gear (about 300rpm roughly) I dipped the clutch and revved
the motor, and it responded well enough, but back into gear and the problem
persists.
Pulling off the motorway onto surface streets, I noticed I had the same
problem in every gear - it would get so far and go no further, top-end power
has been cut off. Otherwise driving great.

I have noticed a slight rattle coming from the exhaust, and upon inspection
last night, noticed the catalytic convertor was very very hot (actually
crackling and spitting) and there appears to be a split in the downpipe as
it meets the cat.
Is this likey to be the problem as regards loss of power? I'm thinking the
cat may have broken up internally and is strangling the engine??

Many thanks in advance.

Dave