So I have a bad cat and can't find a used one and don't feel like spending six hundred on a new one. Can I use a gsr cat with no noticable power difference or smog problems?

Also, I bought the car with a greddy header but I think it gets so hot it's almost melted my fan wire. I have the original header, should I put that back on? I know it's preference but is the header giving me any noticable power boost?

 

The header melted your fan wire? Why don't you just re-route the fan wire away from the header? I have no idea if the Greddy header added power. Probably not much.

The OBD2 GSR cat was a much longer design. So no. I don't beleive that the GSR cat will work. The non GSR cat might be the same part number as the ITR cat, though. Look for one from a LS/RS/GS/SE. When I replaced mine, I don't remember there being a difference between the regular model cat and the ITR cat. You can cross check part numbers at www.hondaautomotiveparts.com

 

You might call Hytech (http://www.hytechexhaust.com/). IIRC their cats were $175 last I checked. Way cheaper than $600...