Without your cat attached, I think you wouldn't have the appropriate back-pressure and that could affect performance. I'm not sure that would be valid test, but I suppose if there was no change (whether positive or negative), it may be elsewhere.

What are your plugs gapped to? Compression check and depression chamber check would be good to look at also.

Did you do a bowl vent valve check? Basically, you plug the hose leading from the charcoal cannister to the bowl vent valve on the carb and see if this helps performance. If it does, it may be that your bowl vent valve on your carb is bad.