Originally this throttle body was on a 1,400+ hp twin turbo motor and I'm assuming the hole in the fin was drilled in order to feed enough air for idle.

I'm currently trying to tune this with holley efi should I replace the fin or try to tune it with this amount or airflow? Does anyone do this or know if this will cause issues tuning/ idling?

I had a carb butterfly drilled for one engine. Then had the hole brazed closed when the carb moved to another engine. This is the first time I've seen this mod on a TPI engine, so I think you might have a hard time tuning for that much extra air. That is going to define your minimum idle speed.