In my BMW 3 Series (F30) the part is at about 25 ° C when the vehicle stops !! dropped after about 3 months little use (which was not the middle of summer !!!)
Total loaded with a weight of about 320 gr incl holder (Proclip, adapter for Navi, Navi about 240 gr).
Conclusion: Your Navi should be much easier or choose another fastening for the Navi.
