I have just release iKatastr2 app on AppStore (free) that might be useful for you to look at as it uses WMS sources and custom tile loading on top of google.
App does simple stuff – shows cadastral information by tapping on cadastral map of the Czech Republic.
As all is driven by JSON configuration it is quite easy to render similar information from other sources.
http://itunes.apple.com/us/app/ikatastr2/id545292674?ls=1&mt=8
enjoy !
∫ ∪ ∧∴ ∋ ∈ ⊇ ∧ 
What are you using for the overlays? Multiple overlays, a single overlay with all the lines or an extra layer that is controlled by you e.g. OpenGL?
Great app btw
thanks, I use simple overlays, I will post soon blog about overlaying WMS on MapKit