That would depend on which interface and drive technologies you use. A SSD connected via Thunderbolt would likely be faster than an internal hard drive.

Not if you get one of these:

http://www.amazon.com/Samsung-Portable-250GB-External-MU-PS2...

This + SSD (Crucial IIRC) gives me 200+Mb/s write, 400+Mb/s read speeds: http://amzn.com/B00FCLG65U

USB 3.0 is pretty fast. Without TRIM performance will probably degrade but that will take time.