I've recently began experimenting with the HID service and connecting to iOS devices in order to achieve lower connection intervals with Apple products (as described in their Bluetooth Accessory Design Guidelines for Apple Products). I am able to successfully connect, and after service discovery, my iPhone 4S (iOS v7.1.1) sends an update request lowering the connection interval to 15ms. So far so good.
However, once the connection is established, it seems as though only 1 notification per event is allowed as opposed to the 6 notifications I am usually able to send when the HID service is not present. I've confirmed this is some restriction with Apple as I am still able to get 6 notifications per event with the HID service when connecting to the Nordic development dongle or another non-Apple device. I have already asked Apple about this and am waiting for a response, but it would be very helpful if anyone knew what is going on in this situation.