I don't totally understand why catatlytic converters are treated as a four letter word. I know they're put there to reduce emissions and they create slightly more back pressure, but do they actually rob an engine of power or make the engine itself, not the exhaust, run hotter?

My take on this...

The converter itself can be setup to add virtually no restriction to gas flow.  Depends on the manufacturer and his budget.

The main problem with CATs is that, to work properly, the engine needs to operate in a very narrow fuel air ratio (yellow band) which is restrictive to performance.

Ideally, to keep the CAT happy, the engine should be running at 14.7:1  (see diagram that shows this is where all the nasties are at a minimum).

Best power is a little richer at around 13:1, and running a little richer takes the CAT outside of its operating range, and eventually can damage it permanently.

Modern (Car) systems have a post CAT Oxygen sensor to check if the CAT was able to do the job properly and the ECU can lean the mixture out if required and/or throw up the fault lamp.

There a lot more to the topic around cold start, full throttle and "Cycle beating"  but thats the basic reason why CATs get a bad rap.

Also have to remember Harleys are air cooled, not water cooled like a Car. One reason why i think Harley will have to water cooled .

I don't think i would run too well with a cat up my back side.