don't think a shift kit would void your warranty if the Interceptor doesn't, and I also don't think an LSD would, because they wouldn't know it was one, and LSD looks exactly the same as the stock diff. and in 3 years the Interceptor may have already ruined your tranny. I think if you took 2 hours of your time to put in a shift kit, you would be much more pleased. Just my opinion though. Without a shift kit the Interceptor is much needed though, otherwise the shifts are really wimpy and mushy.
The Interceptor only takes a minute to remove if I have to bring it in. The shift kit is a little more time consuming but you've probably got a good point that they wouldn't know it'* there likewise for the LSD.
I have the Interceptor adjusted now so it is very easy on the trani and I don't believe it has done any damage.