Hi, firstly, what is the size in mm of the Stillen high boost pulley and will I need to retune with it?

How much smaller is it compared to the stage 3 & 4 pullies? Worthy investment? Any info would be great! The pulley I have on my 2005 VQ35DE is abt 56mm..tks all

 

No idea, what size it is but you'll probably be way over spinning the blower. You also need to think about cooling, that air will be super hot.