I'm curious why you didn't use the stock iTunes app store badge. That might convert slightly better if you get some site traffic.
You should make sure to go through AppCod.es preso on app store SEO. I am regularly amazed at how bad my competitors are on their app keywords. http://www.appcod.es/
You should go sign up with LinkShare and whatever the other iTunes affiliates are and work out your affiliate link from your site. 5% of the sales through your site might not be much now, but it's free money.
You should make sure to go through AppCod.es preso on app store SEO. I am regularly amazed at how bad my competitors are on their app keywords. http://www.appcod.es/
You should go sign up with LinkShare and whatever the other iTunes affiliates are and work out your affiliate link from your site. 5% of the sales through your site might not be much now, but it's free money.