MGC bulkheads give +95% hull HP. Mirrored is the same, but has a damage modifier - your hull takes 25% less damage from thermal weps, and 25% more from kinetic.
If you can deal with the speed loss from the bulkheads, MGCs are a good way to gain some HP without using internal slots. They're expensive though, and their HP to weight ratio is abysmal. 629 HP on a corvette for 60T weight. If you don't need to use your internal slots, buy HRPs instead. A 1D HRP is 110 hull HP for just 2T weight and costs a few thousand credits instead of 170 million. Or use all if you're greedy.
Mirrored bulkheads are like...400 mill? Unless your tactic really needs thermal resistance it's hard to justify bulkheads adding that much rebuy cost given they have an equal weakness on the other end of the stick.
Edit: on your question about "armour and shields"....bulkheads don't touch shields. I'm amazed if you've managed to accumulate corvette level money/rank and were unsure of that
