My cars not coming on boost till 4k its pissing me right off what ways can i reduce that 4 k to say 3k? The cars an rs turbo.

I have a large front mount GRS I/C
superchip
Piper t2 cam
stage 3 head
Lightened fly wheel
4x4 cossie turbo.

I think the main thing is the pistons im running as they have been skimmed to run higher boost

When i had the old standard t3 on it was the same 4k area.

I fitted a pipercross cone on today as well now its more like 5k when the turbo comes on boost

Any help