This would, of course, reward people who level their mons to 35+ or even higher...Love Poliwrath? Not everyone will be willing to level the normal high CP mons past 30 if this change was made (looking at you Rhydon).
Gym meta will change instantly as well and the current top 5 will be replaced by the best 5 actual gym defenders (which sorta makes more sense) since we're collecting a gym "defender" bonus after all. Would help Poliwrath, Slowbro and Exeggutor instantly since their Psychic moves are pretty hard hitting...
That said, I'd assume they will possibly implement a mix of mon level and CP to make it so a lvl 30 won't be dropping random magikarps when they aren't really that strong as a defender to begin with.
This would also hurt lower level players since any high level player can just put a trash mon with bad IVs wild caught (lvl 30) and be above a lot of stronger defenders so if you are level 27 and have a maxed Snorlax, someone with a lvl 30 Karp would place higher than you making it sorta lame too.