Does An External Solid State Drive (SSD) Increase Performance In The Same Manner That An Internal One Would ?
It's definitely worth going internal. Someone asked the question some time ago here and the conclusion was, that even with USB3 (which only the newest Macs have), the internal version is more than 50% faster.

USB3 external will probably still be faster than a regular 2.5" MacBook drive (assuming that you have a MacBook), maybe even with USB2, but with older hardware (and thus older connectors, e.g. just USB2) the margins will get slimmer.

It may depend on your machine (e.g. if you still have warranty or if it is one that is hard to take apart, like the the very new superslim iMacs), but having retrofitted an SSD into a 2009 iMac I can say the benefit does outweigh efforts IMO.

Also given the SSD you linked there, you can get a current Samsung of the same size less expensive and spend the rest of the money for someone skilled to put it into your machine.