thanks guys.
truth is i rarely ever use any armour, and when i do it is just the 1st "improved one". I am just mucking around trying out a few different builds.. change is as good as a rest etc.
as i understand it, i am miles below my "optimal" mass for my thrusters, so, as long as i am understanding it correctly (i may not be!) it wont affect my handling too much.
The agility gain you get caps at
half your optimum thruster mass. So dropping mass will give you increased speed and agility until you are at half the optimum thruster mass, and dropping further below this gives you no boost. When you are exactly at the optimum thruster mass, you get the "reference" ship speed and handling - so for example, the Vulture's stats on the wiki, coriolis etc. state a "speed" of 210 m/s. This is the speed you get at optimum thruster mass, and what the devs call the base flight model.
At half optimum thruster mass, the Vulture hits 244 m/s. It's clearly not double the speed/handling but significant, which does mean that as you have pointed out changes under the optimum mass don't have as drastic an impact as you first think.
For balancing, larger ships tend to be able to fit MGC bulkheads and not really give a crap. Even though you are correct in what I mentioned above, smaller ships typically want to avoid MGC bulkheads as they're usually quite over half the optimum mass even with conservative outfitting. The mass from MGC bulkheads
will affect their handling, but give very little hull HP boost. A few HRPs (low class where possible) will give much better results.
It's also worth noting that because the default lightweight bulkheads have no additional mass, you can RNGineer these using the reinforced mod for more HP/Resistances, and not gain a single bit of mass. So if you want more hull bulkiness without the associated mass increase, a high grade mod to lightweight bulkheads is a really good compromise.