This shouldn't add any overhead
Oh, the runtime CPU and memory cost would be tiny, I'm sure. But now, as a code reader, I must scan past three new globals to see the meatier parts such as pindexBest right above them. It's not that I don't like the feature, or even (maybe) enabling it by default, but adding things in this way gradually makes the program harder to understand. I'm assuming mere integer read/set doesn't need a mutex to be safe-- you either see the value before or after the change.
Even for uint64 on 32-bit systems? And what if parts of the same getinfo result reflect changes that other parts don't reflect?
but now seems a good time to mention refactoring options.