What's the recommended comp ratio for a 1.8 zetec running a t3 on emerald K3 ECU?

I found some pistons which will more or less fit the standard rods, and will lower comp ratio to around 9.0:1 without me having to spend £600

Is this low enough? or am i advised to go for a thicker headgasket too to shave a couple points off?

I know boost usually means lower CR but i know some people run 'high' CR with boost too, which is fine aslong as it's mapped well...

I'm aiming for around 180ish bhp and decent torques/off boost response.

I'll have 3 maps on the fly with the K3.