The unit inside the vehicle doesn't affect its performance.
However, units inside will gain XP faster typically since vehicles tend to inflict damage against higher XP units.
The unit does however effect the vehicles preformance. The unit when on foot has a low threshold to be vet 3 however when put into a vehicle although that unit is vet 3 his overall experience maybe only be enough to vet 1 a sherman because of the different xp thresholds for vetrancy of the unit on foot and the unit operating a sherman. That being said the xp sticks with that crew member. For instance I usually make 1 regular sherman then I bring out the easy 8's. I will then take a 1 star or two star reg sherman crew and move him into the easy 8 to have a 1 star easy 8 off the bat. This works with all vehicles like say a stuart crew into a sherman but you have the mind that the xp threshold is different for every vehicle. A 3 star m8a1 howie crew might only equal a 1 star sherman crew due to xp differences.
So to me vehicle crews are best left to remain inside the vehicles as for the thompson upgrade the only time I could see you would want to use that is if you know for sure your vehicle is screwed you could possibly eject your crew before it is destroyed and then retreat them and make them into a inf squad but from what I hear there still not great at it. Also if you were capturing a point with your crew they have to be outside to capture it so that would give them a little more firepower i guess while capturing but a sherman needs 3 to operate it, if two die quickly you cannot jump back in your sherman and the enemy can if they have 3 or more so that is highly dangerous