It seems counter intuitive but I think you are right. From everything I have read the 20MHz width is mostly for legacy and should on paper be slower.
I don't have a solid way to test the actual transfer rate other than one of those online speed test (which I'm sure aren't a very accurate way to measure this), however its is telling me that my connection to the internet is significantly faster. My signal quality also got a hefty boost and that may be the real cause of the better connection speed.
My speedtest readings have gone from 5-7mbps during heavy traffic times to 12-15mbps.
durring slow traffic times I'm capping out at nearly 30mbps.
As I said I don't think this is a really accurate way to gauge. Do you use a program to monitor the transmission rate of your 825 to your 1522?