Its hard to measure how much of an effect implementing canonicalization has. I've done it an a couple sites and I'd say the sites gain 5-10% traffic over the next couple months usually.
It can have the biggest impact when googlebot is spending so much time crawling non-canonical pages that it doesn't get to all of your content. I'd guess that 1 million posts is at least 100,000 forum topics. That is a lot of content for a PR4 site. I'd imagine that googlebot's crawl budget isn't enough to regularly crawl all of your content. Increased crawl rates could lead to better indexing in the long tail for you.