http://www.worldstadiums.com/stadium_menu/stadium_news/general/articles/2003/art0042.shtml Personally, I think its pretty silly and I don't buy into it holding much or making a huge difference especially in MLS, but I thought I'd show you guys. BTW, its extremely short.
REUTERS - It may please the fans and bring in bigger crowds but moving to a swanky new sports stadium could have a negative impact on a team's performance. A study of professional baseball, basketball and ice hockey teams in the United States that moved grounds between 1987 and 2001 showed they didn't do as well in their new home. "Teams that moved lost an average of 24 percent of their home advantage," New Scientist magazine said Wednesday. It may not sound like a lot but it could make a dramatic difference in performance, it added. That's it right there. Again, its not even specifically talking about MLS, but I thought I'd show you guys anyway.
IMO, new baseball parks have had a much bigger impact because the old stadiums the teams might have played in were too larger and were more for football and the new ones are an attraction unto themselves. Jacobs Field in Cleveland has to be the best example of how that stadium made that team better, but then again there's Comerica Park in Detroit.