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+1 regarding that's not necessary to change the domain structure over just a little higher PA. The way it is now is just fine. Although, i do not agree with that best practice that Martin is saying. I do not believe that there is no gain with www o non-www. The difference could be in the users/visitors in any particular case. Best Luck! GR.

@Ressler - thanks for weighing in. My gut instinct is the same as yours regarding the exact-match approach. To further support the argument from a customer perspective - it may also benefit future Adwords Marketing or Link Building strategies to be able to read it as one piece. Any other opinions out there?

Yes, all good insight. We are the developers/agency for the client...and we are only considering this option purely on budget reasons. There is a lot of work to address the move to a new CMS, we require stronger content with the move and on top of it all we have to integrate all content to the new CMS....this equates to a treamedeous amount of work...not the best argument but again, posting this in seomoz allows us to have a nice sounding board. The current CMS is so poorly geared for today's seo strategies that migration to another CMS is 100% required. Moving hosts isn't the problem. The CMS is. To illustrate how pour the current CMS is, we can't even touch the root .htaccess file to redirect WWW. Doing so breaks the entire CMS system, insane. Thus we have www.mysite.com and mysite.com directly competing against each other, duplicate content etc. That's just 1 issue of many. Moving to a new CMS is required, but with this move we have budget constraints wrapped in a very big site, wrapped around seo migration issues, wrapped in making things painless for the client, wrapped in making this all work. We're looking for any insights knowing very well best practices...but having to deal with the reality of budgets. This could end up being "save a penny today, costs big bucks later". We understand this is our unique  issue and we may have to bite the bullet a bit, go with something like Cart66 and work through the bugs knowing the light at the end of the tunnel will be a brilliant seo/business solution for the client...but may take some painful hours getting there...hours we may have to suck up to keep a happy client and a relationship we've nurtured for some years.