I had the stock Nismo, persevered with it and it did get slightly, not much, deeper. Couldn't live with it anymore, the GRiP is pretty meaty, I would go as far as to say better than the Akrapovic/Berks combo I had on my last 370.
Haven't heard a Stillen system in the flesh but I know Sipar is happy with his
Suits has a video somewhere, that convinced me. I nearly went for the Armytrix system, but couldn't justify the cost.