Best Premiere Project set up for my rig

Hi -

Just bought a Macbook Pro Retina and need to get working on a project straight away.

There is lots of footage which i am having to store on an external USB 3.0 HD, along with my Project location, as its too much for my 512GB SSD, but i am wondering if i should set my Scratch Disks for the Project on my SSD, will this speed things up?

I realise the USB 3.0 HD is a bottleneck and will be purchasing a 4TB Thunderbolt drive to work from soon, at which point will I still want to keep the Scratch Disks on my 512 SSD, or move those to the external Thunderbolt drive too?

Thanks in advance