I think it would help, if you are selling advertising direct to advertisers. Quantcast justifies the prices you set.
As for declining impressions, you may want to check across all the major browsers to make sure there aren't conflicts between the quantcast and AS codes that is preventing the AS ads from displaying properly.
Check your log file and see if the percentage of decreased AS impressions matches one of the percentage of one of the browser types visiting your site. Then check that browser type if you have it available.