In my experience dynamic urls are NEVER spidered as heavily or ranked as high as static pages by google. For one thing, dynamic pages require a lot more server resources to load, and google does not want to risk causing a resource overload or database failure due to spidering it too heavily. We offer a
url rewrite seo mod for x-cart and our clients have frequently seen a 400% or more increase in traffic a couple months after install.
In my experience, top things to address to rank well include:
1) Static Url
2) Custom Page Title
3) Custom Meta Description
4) Custom Meta Keywords
5) Keyword(s)/phrases in the page text that match 2-4 above
6) Inbound links to this page including the keyword(s)/phrase
7) Internal links to this page including the keyword(s)/phrase