Hi there. Based on my own recent experience, when I applied for PR under Spouse Sponsored category, I included an application to extend visitor visa. Then 2 weeks later, CPC wrote me a letter saying that my visitor status is expired so I need to apply for restoration of status as visitor. Instead of $75 extension fee, they requested me to pay $200 restoration fee. Yes, time is crucial because based on the CPC letter which they gave me, you're only given 90 days after visitor status expiry date to restore your status as visitor. They reiterated that visitor status expires 6 months after you entered Canada. In my case, I have a visitor visa sticker that is pasted on my passport which is valid until 2014 so I thought I am allowed to stay here until 2014. But I was wrong because CPC letter told me that implied expiry date of visitor status is only 6 months after arrival date in Canada.
One more thing. I also heard about implied status thing but I didn't trust that. It's always good and for your peace of mind, to have a real visitor status, while waiting for a PR AIP, so better pay $200 than be worried everyday about living in a country without authorization in paper. HTH!