I don't think it pulls harder above 6k, but the acceleration under 6k def makes the car feel faster .. No way of knowing unless I would've done back to back pulls to compare manifolds. With the gt350 manifold it pulled to 7700ish.
I had a gt350 mani with long tubes.. Absolute beast over 6k however under that, which is where the car is driven most, was less then impressive. Sold the mani and am running long tubes with k&n filter and tune and car pulls a lot harder. My opinion though.
I have them with Borla S Type axle backs.. Fitment is ok and the pipes look nice. I didn't intend on doing the mid pipes but the stock midpipes were already fairly corroded and just said hell with it.
Open element intakes generally have higher iats... As far as the shifting, they pretty much told you if it's to harsh get a performance tune rather then the race tune.
Installed these along with other suspension components and my car handles night and day better then stock! Would definitely recommend as one of the first mods to do.