Would a TB SSD be about as fast as an internal SSD?

ssd can not saturate a TB connection. It will be identical in speed as if internal

 

I have an OEM SSD left over from an upgrade in an envoy USB 3.0 enclosure. It is much faster than a rotational drive in an enclosure but the internal benchmarks about twice the external (450MBps internal vs 235MBps external SSD in USB 3.0 enclosure). A high performance rotational drive in a USB3 enclosure is about 120MBps using the same benchmarks. What that means to your work flow, I dunno.

My experience shopping about 3 months ago showed it to be less money to max out the rMBP SSD (768GB) than to purchase an external larger capacity SSD with enclosure. External rotational media is less money.

 

Perhaps right after I buy one ....

I plan to use a TB hub/enclosure like the sonnet echo when its released. It has a number of SATA ports where one could hook up a "commodity" SSD, along with USB3 and some other stuff, including a DVD drive.

In the mean time I'm using a $50 Belkin 4 port USB 3 Hub where I connect my large raid enclosure (it has eSATA, FW and USB3 interfaces), SSD enclosure, printer, and camera and plug the hub into the rMBP with a single USB 3 cable. I use the Jetison app to automatically unmount the drives when I close the lid to sleep (so I can make a quick exit) and remount them when I wake up the rMBP. This hub does not degrade USB3 performance as far as I can tell and integrates with the Mac well. Other hubs have issues.

I also boost my internal memory with a 32GB SD card for day to day trivia storage. You can get these with 128GB, perhaps more if you have the $$$, but they are slow USB2 type speeds unless you spend lots of $$ for something like a Sandisk extreme pro (which claims 95MBps reads). Still slow in comparison to external drives, but much more portable.

I use the general work flow where the OS and apps as well as working data are on the internal SSD, finished product is moved off to the external drive as well as backups. The SSD external is used for my Windows boot (via parallels VM).

 

Before yesterday, an external SSD via Thunderbolt would perform close or near to an internal. After yesterday, Apple is moving over fro SATA 3 to PCIe which is a big jump. 700-800 MB/sec versus 400-500 MB/sec So, the answer today is no.

 

The flipside is the 256GB base SSD is not enough when you really start using the MBPR. Another $300 for a 512GB instead of 256GB base isn't that bad - when I bought mine ~2.5 hours after its initial announcement last year, that upgrade cost $500.

Thunderbolt storage is already expensive. You'll end up paying through the nose for the SSD versions. What you should do instead is buy a HDD version of the external portable TB storage drive, open the unit, and replace the HDD with an aftermarket SSD - this way you can have something more than just the 256GB's in most SSD-based TB drive offerings.

Having that said, TB-based SSDs are limited by SATA3 speed-wise, not TB.