Sacrifice PD for pen? After you get Starheart most PD items are around 50 PD like the Book of Eternity, with Fatalities and a charged Abin Sur's the main exceptions. Oh and maybe Helm of Fate.
Also earlier I was Aegis 4 + Diamond 2 WW during my ult (+50% all armours) and flash still killed me with one rotation. (Edit: and I did NOT sit in his ult for longer than 2 seconds, in particular I avoided the detonation.)