About your "smelly" Viper
well I had the Belanger headers & exhaust w/o cats and had no asphyxiating fumes. This was with MAJOR exhaust leaks, so from that I can only guess your problem is unrelated to leaks unless somehow the leak is going directly into the cabin. Now some people are just more sensitive than others regarding fumes and no cats do create more irritating fumes but again they never bothered me.
Q. Will replacing the mufflers with hi-flo cats help this problem?
A. Yes see above, but what did the people in the 1960s do? I mean there were no cats until the 1970s and somehow they lived.
Q. Will replacing the mufflers with cats offer the necessary "resistance" to the engine compression?
A. ?? have no idea what you mean, should make no difference
Q. Will replacing the mufflers with hi-flo cats alter my HP or dampen the throaty sound I've come to love?
A. Maybe it just depends on what cats & what muffler, you will probably have to just try to find out
Q. Or should I consider BOTH the cats AND the mufflers?
A. That's what most people do