I have been testing this concept myself since November 2007. My main website is here www.McCordWeb.com. I created a spin off site just on the topic of custom web design and our templated Quick Launch web design services here www.McCordWebDesign.com.
Since November I have been monitoring placement and trying out several search engine optimization techniques. What I have found is that my spin off site has never really quite gotten the placement that I thought it would even though it was very focused and on topic. I was careful to not have duplicate content for the site so as to rule out that issue as a factor for organic placement.
Here’s a real world search example:
on www.Google.com search for custom web design Maryland. My main website is #1 out of 304,000. My spin off site is #80 out of 100 results. Although this is an incredibly small sample, when client’s come to me to say let’s do a two page spin off site – like a doorway page, I simply will not do it. It is far better to develop and work on making your main website the best it can be than to fragment your message and try to scam the organic results. Clearly my own test proves that a spin off site, no matter how optimized will not beat your original main well-developed website.