Ship's stats come from both the ship and the pilot.
The things that affect the ship's contribution in stats are: Ship level, period. Nothing else will change how many stats a ship just has from itself.
The contributions from the pilot are determined by Crew rating, which is determined by: Ship stars, Pilot stars, Pilot level, Pilot ability level, Mod tier, and mod level.
Since there are 6 mods, I could see the upgrade to tier 6 giving out somewhere in the neighborhood of 200 ship gp for each mod (though this is entirely a guess). Poe gets 1 speed for every 2 of his G12 pieces which give about 200 gp. Mods could give a ship up to 4 speed if it is one of the faster ones. Poe gets 1 speed for each of his g12+ pieces from the Sith raid. If it's a smaller amount of GP for that mod upgrade then it will give less speed. On one of the slower ships, it will definitely give less speed.