I can't come up with any other ideas, but I can answer re the A050R. They are consistent from new to medium wear, and drop off slowly. Plenty of testing of this on heavy cars 1800kg commodores up here, and the guys are very happy with them, including racing up to an hour straight (a lot longer than any targa stage). I'm not happy with their 600 a tyre price tho.
Very different to the previous A048R which did indeed drop off significantly between brand new and medium wear - in the order of seconds a lap.
Not to say your problem could not be tyres though...depends how old they are, how they were stored, how they were heat cycled when new etc etc.
Also, if you are testing the impact of colour on performance, maybe paint it yellow/purple/silver and find out if it becomes cursed. Either rolls down hills or blows up motor depending on the order of the colours.