The problem is the high "vardiff" that causes small miners like gridseed and usb miners to show incorrect numbers.
I mined on ghash.io while they had bonus, and because of the low difficulty, the khash on their website is much more stable on small units.
Are there any chance that wafflepool will get a lower vardiff than 512??
You just mean "diff", not "vardiff"

Probably not. At least not in the immediate future. We did (just today) update the website to provide 15min hashrates instead of 5min, so your numbers should be _much_ closer to correct.
As has been mentioned many times before, difficulty really doesn't affect anything other than variance.