Considering the awful rap he gets compared to his slightly above average presidency, George W. Bush
ABOVE average? At best he is below average, at worst a failure.
It is impossible to judge Bush's presidency and its effect at this juncture in time, that is why he is underrated. Americans think in the here and now and not the future, that is our problem
in other words, according to you, there is no chance that he is currently being overrated. Bad way of thinking. You said it yourself, history will tell, but I don't see any reason to assume that he will become drastically more popular. Look what happened to Johnson.
About Americans judging presidents and thinking in the "here", wouldn't it be fair to say that many some presidents become artificially overrated, precisely because the public doesn't know very much about them? Look at FDR for example, I think he was what we needed at the time, but lots of people give him more credit than he deserves. Or Kennedy is a great example: although he didn't really accomplish much during his time in office (not his fault, but true anyway), people worship him for some reason.
I don't think you can discredit entirely what current public opinion is, but you're right that it'll most likely change eventually, whether positively or negatively. Look at Truman, he was hated at the time.