Power feels choked below 4k rpm in my STi

Wondering if anyone has had this happen to them and if there are any known fixes? It's to do with a 56 plate Hawkeye Impreza spec D STi with the EJ257 engine, no mods other than RCM head studs and gaskets.

Below 4k rpm, the car feels like it's being choked, with it being more apparent at low oil temperatures. As soon as you hit ~4k the engine picks up and you get a more positive burst of power.

It happens more noticeably when accelerating at a "normal" rate rather than foot down, but still happens with the foot down as well.

The only engine code I have at the moment is P0410 (Secondary air pump) which I've not got round to sorting, but don't see how this would effect the power as it's more of an emissions thing?

Any help is appreciated!

PS it isn't turbo lag before anyone tries to be clever! 😉