Here's a link to an article about impedance effects of vias.

http://www.sigcon.com/Pubs/news/7_04.htm

Dr. Johnson looks specifically at the improvement you can achieve by using lower dielectric constant material and reducing your board thickness. The point he makes is that, at high frequencies (short transition times), vias can become like stubs on transmission lines. It's a pretty good discussion if you want to understand what's going on (a key first step to choosing an appropriate remediation).