id drop the tiamats for a black cleaver/last whisper, caitlyn is meant for tank shredding and having either one of those depending on how high the armor is is a MASSIVE addition to dps, if black cleaver reduces armor by 45 so if someone is at any more than 114 armor youd want to use last whisper 114 or less youd want to use black cleaver, theory last whisper reduces armor by 40% of the current target armor so 114*0.40=45.6 armor reduction so you may as well go with black cleaver cause it does 45 armor as well as attack speed increase. if its anymore than that the 40% gives that LITTLE bit extra armor penetration + bonus AD