Similar thing happened with my T5 mate...bolt was seized solid and tried everything to get it to move.
Quick fix, while the engine was still in the scoot, was to just partially drill the bolt and then put a tap in it to enable me to secure my exhaust (worked great).
As soon as I had the engine out at a later date though I drilled the f*cker out as it was p*ssing me off knowing it was still in there
I just worked up in bit size until the seized bolt collapsed. Took about 20 mins.
Don't forget to lube the new exhaust bolt when you replace it to prevent it seizing again
