From what I can tell, the biggest issue at South Lakes at this point is the IB program (vs. AP). The redistrict that happened a few years ago has changed the make-up of the student body a bit and scores are going up. My daughter's preschool friend has older brothers at South Lakes and they are very happy with the school. People hold on to old stereotypes for a LONG time when it comes to schools.
this should be helpful to you. south lakes sat scores are quickly closing the gap with the other pyramids you mentioned.
http://www.city-data.com/forum/northern-virginia/1697440-2012-sat-scores-updated-w-loudoun.html
I just thought of one more suggestion for you - annandale in the woodson district. The house will likely be older, smaller and need updating, but it wouldn't be far from alexandria.