IV used to be a very important factor during the early release of Pogo. But at this point in time, when almost everyone has 25+ account, this became almost irrelevant. I have not seen a pokemon that cannot be beaten, or a gym that cannot be taken, no matter how strong they are. I spent time populating a gym with 10 level 30+ pokemons and almost perfect IV,s and it was taken over by the time that I get home. And vice versa, I have taken a level 10 gym with 30+ level pokemons with one of my level 27 account, of course I was loaded with revive and lots of potions.
Starmie's max CP is only 2182, it will not stand a chance against a same level grass pokemon, Not to mention that powering up an exxegutor is way cheaper than powering up an evolved starmie. But not saying that you cant beat a lot of pokemons too. Defenders are always slow and movement is predictable.
But should you keep it? Keep it, higher IVs evolved stronger with higher CP. But look at the attack/defense/stamina property too, so you can decide what purpose it is going to serve for you.