i've just started today my first experiments with phonegap and web frameworks like jquerymobile and sencha and im not so optimistic about huge performance improvement: it's basically just running the webpages of your app from the smartphone memory instead of loading it from the website. NB of course it all depends on the way you develop the app (background updates etc..) but i've quickly tried a long listview in sencha touch vs a native one and the first one was at worst not shown correctly in the smartphone browser (opera) and at best not as responsive as the native one.

disclaimer: these was just preliminary tests and im not a real coder :)