Just loosen up the pipes at the exhaust manifolds and see if it runs OK then.
Don't leave it like that, just drive it long enough to tell if it's fixed, and have the windows rolled down. It'll be loud, and you can get exhaust fumes into the cabin, so please, just see if loosening up the pipes fix it, that's all, this is NOT a repair!
ps: You'll feel exhaust coming out at idle, and it's idling fine, right? The problem is when you bring the rpms up under load, if the cat is plugging up, it won't flow ENOUGH exhaust under those conditions and you build up too much backpressure and have poor scavenging on the exhaust stroke= no power.