Drive speeds ... PCIe vs external thunderbolt SSD
The 900MB speed is realistic, this is achieved on a few of apple's new mac offerings. 
An external Thunderbolt SSD would not be as fast, the only way to make it the same speed (or possibly faster) is if you have a multidrive thunderbolt enclosure running in something like Raid-0 with SSDs in all slots. 


The prior enclosure would be rather expensive though. It is probably best to have as much of that internal storage as possible as that is not easily upgradable. I would stay with the 1TB model more internal storage is always better.