My car is doing the EXACT same thing. Manifolds were glowing bright red, but strangely the cat wasnt glowing at all. A few other posts strongly point to the OPTI messing up timing enough to cause some major amounts of fuel to burn up in the manifolds... Plz get back to this thread once you do see what happens
Mine started doing this last night, can barely get above 20mph. I have an opti on the way...

 

Any updates?

 

Also check for pinched or bent fuel lines. I say clogged cat, but replace the fuel fitler first, then if that doesnt fix it, unbolt the exhaust before the cat, or just cut it off. won't matter since you will need a new cat anyway.