My understanding is that the Heavy Breather ac doesn't have a significant contribution over the stage 1 until you reach a stage III engine upgrade with head swap.

Personally, I kept the stage one since I decided to not do the 103 upgrade until my extended warranty runs out. (Harley warranty only covers engine performance upgrades when done within the first 60 days of the bike purchase).

I like the uniqueness of the Lo's black football AC and the "Fat Boy" on it's medallion insert, so I kept the stock cover.