I am thinking about buying an ssd to hold some of my games on xb1 (mainly for gta). I know it improves load time into the actual game it's self. But does it also improve load time in and out of race lobbies, mission lobbies etc...?
Yes, it will speed up. Any time the game has to load something, such as a new cutscene, load in to a new map, boot up the game, etc, it will be much faster with an SSD over an HDD. You can search YouTube for "SSD vs HDD in games" for a bunch of videos comparing them side by side. The difference is amazing. Once you have an SSD you won't want to go back to an HDD as your main storage device.