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So my question is going that route asking for trouble (billet).
I'm not sure. I wouldn't think a hub would be that difficult to make. I don't know the amount of research that went into it... nor do I know the amount of work that went into the Mahle Porsche hubs. That being said I do know the Porsche hub has been proven (race and street) for over 40 years in the same basic configuration so I'm fairly certain that "isn't" asking for trouble.
I want to reiterate, for a 4-lug car, this is the only way to go. They seem to be fairly stout and the concept is well thought out; "get vented rotors without buying a bunch of expensive wheels -or- get vented rotors with an original 914-4 look." You'll still need to purchase M-Calipers. This will still save you money because 5-lug Fuchs can be very expensive these days.
For a 5-lug conversion, I still feel the 911 strut is the best buy. You should be able to easily find a set of struts "loaded (both calipers and inserts) for the price of what I've seen the hub kits for. That would make a true 911 strut (loaded) much less expensive than a hub package and a separate M-Caliper purchase.
Regarding your last post; I don't think a 4-lug vented hub would exert any more pressure on a 914 spindle than a 914 rotor/hub combo would. Again, all of this would depend on the wheel/tire package chosen but as you've mentioned... there's no real record of excessive spindle failure.
I’m not panning on these things I’m just making aware that in the 5-lug configuration, there are more options that will end up being less money.