Just discovered Option + Escape by browsing through the XCode Preferences Hotkeys section. It actually lists the parameter info in a tooltip (no other hotkey I’ve seen so far does this!), but isn’t quite as friendly as Visual Studio’s Ctrl+Shift+Space function. For instance, the text selection cursor (caret) must be on the function name (not on a parameter to that function).
Having explored the XCode hotkeys exhaustively, I’m going to assume that this is the best it gets in XCode.