One of my websites is almost static in content and has a fairly constant stream of visitors, mainly from referring sites and the search engines. In the last months I have played around with some options to increase page load speed. I constantly monitored the page load speed mentioned in Google Webmaster Tools, and at the same time looked at the number of pages per visitor. The sample size of visitors we talk about is about 100,000 unique visitors per month, over a period of five months.
Over these five months I have seen a fairly consistent relation between the two. The faster the page loads, the more pages a visitor wants to view. My figures:
4.7 secs: 1.51 pages per visitor 3.0 secs: 1.56 pages per visitor 2.7 secs: 1.58 pages per visitor
I would agree with this theory although I don't have the sample size you do.
People have incredibly low patience and tolerance for slow sites these days. If they can just click-click-click through pages they will keep searching for what they want. If every click is costing them 2+ extra seconds they will get bored if the page they are on is not directly relevant to their search.
I've definately found that faster page load speeds like the improvements you have done will contribute to a better page rank in google! (People happy = Google happy)
The main reduction of load time was caused by combining many images to one. This site has a menu bar which originally consisted of one image per selection. All these images have now been combined in one large image with a client-side map to link areas of that large image to different URLs.