Change shortcut in Keynote
You can do this in Mavericks pretty easily (I just did).

Go to System Preferences -> Keyboard -> Shortcuts, select "App Shortcuts" and click the + button. You'll be asked to select the app you want to create a shortcut for (Keynote, in my case), the exact text of the menu item you want to create a new shortcut for ("Play Slideshow" in my case), and the keyboard shortcut you want to assign (Shift-F5 in my case, which is what the Logitech R400 uses for "start slideshow").