I'm not on a retina display but that doesn't look as nice. The letters are so thin they start to disappear even before using the light-purple-on-dark-purple font.
Judging by my windows machine and using just my eyes, yes, it's a little thinner, and no, there's no problem reading it. The faded-grey-ness of the words "to Heroku" is a much bigger legibility problem than the minor change in font weight.
