in a word ... YES if you switch from 800 to 200K+ overnight. How long have these 800 pages been there? How long did it take you to reach 800 pages? If the site is 'old' and has remained the same for some time, it's even riskier to go from 800 to 200K+

Before going into this any further, could I ask you if you considered launching the new site on a new domain. There are many warnings going up when I read your post. 40K visitors a month is a nice figure and I would be worried about potential problems if my whole income was based on this site. Do the 40K visitors all come from SE?

If you haven't considered launching on a new domain, why not?Is the domain name of the 'old' site that good? You could get both sites running at the same time and - as long as you don't have 800 pages of exact same content - link from the old site to the new saying something like - check out our new website.

Other questions I wonder about are: does the CMS provide search engine friendly URLs? Are you able to check the 301 beforehand (on a test server)? How fast do you want to get the 200K+ pages out?