'You are so adjective' is common in both Br and Am English, but the adjective isn't usually a participle: 'The food is so ready', rather than 'the food is so cooked'.
But recently (I'd say in the last 10-15 years) this restriction has been flouted. The first time I noticed it was in a film when a person at the desk of a drive-thru burger joint said 'You are so busted' (I've forgotten the details, but I think they involved either sexual or marital infidelity on the part of the driver.)
'You are so dead' isn't such a clear case of a participle, but it sounds to me similar. What it means, in this context, is an emphatic confirmation of 'Cathy will kill me'.
Sorry, I can't provide examples (not credible ones anyway!) as it's not an idiom I'd be comfortable using.
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