I wont vouch for either of the cases. Trust me, if there's only one Uncertain thing in the world regarding processing times, it'll be CIC
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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Though I did my medicals after they asked me to do; I got my PP request only in the 10th month and by the time I got my VISA, I only had 3 weeks left to land. So it's not just the money, but the landing time window you are planning to give yourselves.
And I know one of my close friend, who too did medicals when CIC asked, but the processing delayed and ended up doing second round of medicals for him and his wife.
More discussion had already been taken place in this thread. Refer this please...
http://www.canadavisa.com/canada-immigration-discussion-board/upfront-medical-report-t138995.0.html