Lots of good advice in here already, so I'm probably just repeating what others have said.
My 2c. If you are chasing lap time, I'd strongly recommend starting with the tyres first. Street tyres will want a different suspension setup, compared to semi's, compared to full slicks which all want something different.
Once you've got the best track tyres your budget and class of racing will allow, then I would go about extracting the maximum performance from the tyres by setting up the suspension for what the tyres want.