@Verbatos you have my thoughts correct, but if you had a speedometer telling you you are moving in the x direction at a rate of 150ms and also reading 150ms in the y direction you are most certainly moving at 212.1ms in the xy direction. That is simple geometry. The big question is how the game is coded.
its a vector, and the lenght of the vector of your ship will not go past 150m/s whatever happens, if you have thrusters that are angled relative to the forward of your ship, they wont be as efficient for going forward but when needed they can give sideway thrust
even if your ship could theoretically go 300m/s, it wont
another way to see it, since we cant go past 150 m/s, your result of 212 cannot happen in this game, so lets do it backward, on the xy direction you are going 150, which mean on the x you're going somewhere around 106 and same for the y axis (that is if you have an angle of 45 degree from the x axis)
it is indeed simple geometry and basic logical thinking