An extra 5 PSI of boost pressure (20 instead of 15) multiplied by the ZD30 Compression ratio of 17.9:1 means you are now banging off at significantly higher chamber pressures of maybe mid 600's instead of around 550.
Up to you, cobber but I was pretty sharpish in winding my VNT down. I did, however see higher spikes on mine (22) than you have recorded so far.