Bob, I would have to say you are spot on here! Just to add a few angles for thought:-
Most period race cars have ended up racing throughout the 70's and 80's in club events and effectively lost all their proper 'period' spec and patina anyway. IMO the only cars worth retaining full period patina are the 'time warp' cars we all dream of finding that have been locked in a barn for 40 years. Many cars (for example just like Bob's own car) require complete restoration to return them to the glory of their former days reusing as many original parts and features from the car as is possible and safe to do. Not all GTAs, (1300 or 1600) can be put under 1 bracket here; road cars and race cars are 2 completely different kettles of fish.
The value in race cars is down to a few key things:-
1) History (but its only the REALLY famous cars that can gain here - well know AD cars, Horst Kwech, Alec Mildren etc.), cars with minor race history are of little more value than ex-Stradale cars.
2)Presentation - body, correct wing profiles, race build details, paint etc. Beautifully prepared race cars make good money and sell easily!
3) Speed, fast (legal) cars that are well known to be race winners make big money as people are always prepared to buy into the speed.
So where are these values in Europe today? Well, in reality judging by the way the market has moved in the past few years and months you get the following:- a 1600 GTA 'race car' for £65000-70000, but in reality it will need work to bring it to a high level of presentation and speed. The very best race cars would make £90,000+ in today's market - the same as buying a £70,000 car and rebuilding it properly. I would not be at all surprised if someone paid over £100,000 soon for a championship winning car in excellent condition.
Road cars, road cars..... much more tricky than race cars IMO. The key to a 1600 GTA road car is its originality - bits race cars don't need. This is also reflected in chassis numbers - 613 and 752 cars make more money that 848 cars, presumably because of the 2-bolt cross member, 1600 GTA upright and Dunlop brake spec that the earlier cars had. This is obviously seen as the 'true' 1600 GTA spec, as well as the only spec eligable for FIA App K. races. 1600 GTA road cars often miss correct parts as bits have been nicked off them over the years or lost during restorations, good examples of these are GTA uprights, the thin stainless trim up the back of the door plexi, thin stainless trims on door cards and the thin stainless trim at the back of the boot aperture - the list is endless..... Cars missing these parts (or many others) to me lack much of the appeal of the cars and such parts are virtually unobtainable at the same time, yes repro uprights are available in Italy, but they are not very nice. Confusion then arises when a really lovely road car sells for strong money, then everyone thinks their car is worth that much, many not knowing what bits are missing from their cars themselves! Then average cars start being bought for big money (either due to lack of knowledge or due to a lack of other cars on the market), these cars then require much time, effort and money to be spent to make them perfect. Then owners of such cars see that they have put large amounts of money into these cars, as a result you find that the price bracket then moves up.
At the end of the day the value of these cars is what someone is prepared to pay for it due to their desirability and rarity, when this becomes public knowledge the price bracket moves again.
Its tricky right now, over valued average cars come up reasonably frequently, and every so often a cracker comes up that makes big money -that seems to (rightly or wrongly) reset the market value. For sure without intimate knowledge of the cars (or a bottomless account to remove concern) it is easy to fall into trouble, but then when the value continues to rise you get to a point where you could say "well thats what you pay for that condition". Only a few years ago you would have bought the very, very best for £40,000, now that doesn't get perfect cars by some way... The difference between the very best most original raod cars and very average cars - 50% of the value of the very best is the value of incomplete cars.