This section is under discussion.
Thruster blocks increase your ship's thrusting capabilities, allowing it to accelerate and decelerate in a smaller amount of time. The more total mass your ship has (including the thrusters!!), the larger the amount of thrusters you will need to get the same results.
They do not increase your maximum speed though, that value can be configured in your server.cfg, found in the StarMade root folder, it is 50km/h (13.89 block/s) by default.
Thrusters benefit from grouping the way most blocks do, their performance is based on the cubic size of a whole section of thrusters, so the most block-efficient section would be one that goes along three edges of a cubic shape. (screenshot pending)