Im installing holset hy35 for my dohc zc and it has 2.5" 90 elbow from turbo and i got 2" intercooler hotside and 2.25" coldside, and 2.25" piping ic to tb. How much it will affect to performance if i put 2.5" to 2" silicone between turbo and ic?

 

minimal...
wouldn't even worry about that .25" .
might even spool up a bit quicker, but i don't think it will affect performance at all..

 

Turn up ze boost

 

you will be fine, nothing wrong with doing that.