3.5gb of "USABLE" memory on 970 mini?
Simply monitor the games and take a look at the memory usage. The problem is, that the game mostly finds a 4GB card inside and this 970 will run at 3.5 GB in an kind of overflow, because the dramatically increased latency is not awaited. The result is well-known and you'll get a short stutter (or more). I've tried it with 970 and 980 and it is enough to play with the settings in GTA V. Simply increase the memory usage step-by-step. You'll get the moment, if the 970 begins to stutter and the 980 not. Ok, this is a constructed example but a good demonstration. And to be honest, it is not only a memory thing (ROPs etc.) and the driver by himself is trying to use the first, faster memory blocks.
You ever read a bashing review or comment from me about and against 970? Never! But it must be allowed to talk about facts and the reality. It is really off-topic to discuss about NV's kind of communication and the data in the sheets (they are really wrong until today).
And please read the conclusion again. You'll find also a conclusion for GTX 970 Mini. If you read it carefully and emotionless, you will understand my more or less diplomatic point of view. It is a very good card for the money, no question. But it also has some disadvantages.
I'm not a fanboy, I'm simply to old for this shit