Did you also cut open the air filter lid? That was a very big improvement for me, I noticed an immediate decrease in throttle response time.
I also did the air foil thing; some would doubt if it helps, but it does smooth out air in the throttle body. I'm sure you've looked in there and the air just crashes instead of flowing. I got mine off eBay for $32, not a bad investment and the exhaust tone was noticeably louder, don't know if that means anything.
I have a Flowmaster catback, which has gotten louder since I've installed it and it really screams now. Even my co-workers compliment me on the sound quality of it.
From the reading that I've done, I guess the two most restrictive things for the L98 are:
exhaust
computer chip
I had custom chip burned for me by Jeff and you can really feel the difference, especially on the higher end. It was also programmed to keep the engine cooler via turning the fans on earlier.
I would venture to say that maybe doing the exhaust would be the best overall mod you do because not only can you feel it a little bit, but you can also hear it and it'll bring a

to your face!
Might get roller rockers this winter and that's going to be it for me. Headers are just too expensive and the miniram would take me months of scrimping and saving.