While new here, I feel uniquely qualified to make comments, given that I have a tii, an e30 m3 and early 911s...
I don't think the tii can be considered a mass produced econobox... especially not in the round tail light guise... especially if we're talking about the US market. If anything, I think the old 911s benefit from the constant evolution of the model. An old 911 still looks like a newer 911, or at least there is a strong family resemblance.. such that an old one has a ton of character in the ever rising cost of new ones. In the sense that "anyone with money can go buy a new one"... you have to be an enthusiast "and have taste" to buy a older one for the same money. A line from the movie "gone in 60 seconds" comes to mind. I don't think my early 911s perform meaningfully better than the tii... granted, I don't push either to their limits.
I think stock tiis have a good chance of matching of even exceeding the e30 m3... the e30 m3 guys think there are about 1000 cars left in the US.. and I bet that number is similar for tiis.... While the e30 is iconic with the sexy (imho) boxed flares, the 73 is definitively a classic, while the 1988 e30 is not quite old enough yet (again, imho).
As far as cars go.. as much as I love the tii, the e30 m3 and my longhoods.... objectively, compared with newer cars, they're just not that good as being cars... they're slow, not very safe... not fuel efficient.. I have to warm them up before I really go anywhere... but I choose to keep these cars instead of getting newer ones, because they have a ton of character, and that they are BMW and Porsche's hertitage and roots. If the 1965-1973 911 was not as good as it was, we would not have the gt3's today... If the 02 and the tii's were garbage, we wouldn't have all the versions of the m3's that have come around.
I think based on shear numbers, the tii will be going up in value, presuming the economy doesn't take a nose dive. They are beautiful, still fun, and a meaningful part of BMW's success story in the USA. There are certainly better investments out there, but few that put a smile on your face as these do... and if you can make a few bucks on your hobby... more power to us.