The enemy of all electronic components is excessive heat, keep them reasonably cool and they will outlast their hotter running counterparts.
As an electronics industry refugee, I understand completely. That's why after searching for info on VAF, I asked the alternator manufacturer. I figured his perspective from warranty return rate for diode failures would most likely determine his answer.
Also forgot to mention in my original post, my alternator is externally regulated, with the regulator mounted to the firewall. The only electronics inside the alternator, are the diodes.
Anyone running an internally regulated alternator needs to determine if external cooling is needed in that case. YMMV.
One other bit of interesting info. The alternators on both my cars (Honda Accord, Ford Ranger) have similar louvered areas on the back end, over the diode heat sink. I can't say whether they are internally or externally regulated. In both cases the alternators have a higher capacity (and most likely a higher normal operating load, therefor, higher diode heat generation) than my RV. Neither one has any type of external cooling added. Now admittedly, they both sit near the front of the engine compartment with some exposure to airflow past the radiator, as opposed to under the baffles of the Lycoming with airflow off a hot cylinder. And neither one has an exhaust pipe passing approximately 6 inches behind it as the RV does. Maybe the RV diodes will run hotter on the ground. On the other hand, the RV will be operating at much higher altitudes with lower outside ambient temperatures and lower loads, so may run cooler there. Short of measuring diode junction temperatures in a car vs. an RV under varying operating conditions, it is difficult to know for sure.
If the diodes outlive the alternator brushes wearing out, mission accomplished.