From what I know about them the insert is for when/if you don't have a tune. You can take it out w/ a tune and supposedly get better power, so I find it a little hard to believe better times with it in. It seems like any restriction in the intake system would definitely hurt high rpm power.
I had the same intake from Roush and if you take the insert out without a tune it gives the MAF sensor a bad reading. You will end up running the engine lean without the tune so leave it in.