Update - got the SSD drive today. And yeah, there is a heck of a difference.
A few tests I ran. Again, this is the
Samsung T1 External SSD - 500 GB. Ran each test twice, listing the best number received. Scenarios on each test were identical.
MORTAL KOMBAT X
Internal storage - Fade up on copyright screen to being able to skip to start - 0:20
SSD - Fade up on copyright screen to being able to skip to start - 0:19
5% decrease
Internal storage - Single Fight vs. CPU - from loading bar appearing to match fading in - 0:11
SSD - Single Fight vs. CPU - from loading bar appearing to match fading in - 0:09
18% decrease
GTA V
Internal storage - "Loading story mode" screen to opening cutscene - 01:02
SSD - "Loading story mode" screen to opening cutscene - 0:22
65% decrease
The Witcher 3
Internal storage - Continue option on title screen to playable - 01:15
SSD - Continue option on title screen to playable - 0:48
36% decrease
Note that transitions between menus also were faster, which can save an extra few seconds here and there. The initial loading splash screen you get when you launch a game from the dashboard was also noticeably faster to finish loading.
I kind of wanted to run a few more tests but I was impressed enough to stop there. Stuff that isn't too massive like MKX saw meager increases, but there was a MASSIVE difference for larger stuff like GTA V and The Witcher 3. I've read elsewhere that an SSD also noticably improves the performance of Fallout 4 on the X1 as it can load things on the fly more quickly. One thing to keep in mind is the non-SSD USB 3.0 drives that are significantly cheaper will probably return a close improvement to the smaller stuff like MKX (ie 1 second faster vs 2 seconds faster), but not the big stuff like GTA.
So, if you don't play a lot of singleplayer games (not much point in loading faster if you're just going to be waiting for others) and/or you don't mind load times as much, obviously YMMV. To me, the money was well spent.
Yeah, kind of sucks to go from 2.5 TB total down to 1 TB total, but I'm really only playing 3 or 4 games at a time so I can definitely live with it until external 1 - 2 TB SSD drives drop in price.