The issue with the extra engine on Poe's X-Wing comes down to whether it's needed for the story and it makes some kind of internal sense. Is it there just to look cool? Why does Poe need a "boost"? In the Empire magazine picture, the engine exhaust is a different color than the pink glow from the X-Wing's engines, so it seems it's doing more than just adding extra speed or power. The ship is flying directly at the new star destroyer, so maybe it provides some sort of unique thrust that allows Poe to penetrate the star destroyer's shields. Who knows. It all comes down to the story. If people like the story, it doesn't matter how goofy the rationale for a strap-on booster is. If the story sucks, people will say the booster ruined their childhoods.
My theory is that that giant Dorito chip Star Destroyer with the giant guns jumps into the system and the Resistance sees it and goes "OMG what in the Sith is that? Get that monstrosity out of my view! Gah it's like Jabba the Hutt fresh out of the shower ugly! Kill it as quickly as possible so strap on those boosters and all that." :lol