The way this works is important because all those other incurred bonuses result is reduced by the Stacking Penalty just before the Leadership skill bonus at 5% per skill point modifier is applied. This means that an extra skill point Leader has proportionally greater benefit than a similar percentage bonus in other areas before they are reduced by the Stacking Penalty modifier. So an extra point of skill at 5% is worth more than an extra level of Mission efficiency also at 5%. Similarly 2 extra points of Leadership skill at 10% has a slightly better effect than another 10% bonus such as base proximity.
This is also not true. Modifiers are multiplied and multiplication is associative. 3*4*5 is the same as 5*4*3.
No the way it works is 1.3 x 1.2 = 1.56 x 1.1 = 1.716 x 1.1 = 1.8876, rounded down to 1.887 by the stacking penalty percentage = #.## now multiply by Leadership skill value equals final amount of efficiency.
So lets say its 4 INTs with a 3 skill Leader, result = 1.32132 rounded to 1.321 times 1.15 equals 1.519
Lets say its with 15% mission efficiency and a 4 skill Leader = 1.518 (Don't forget about the rounding down to the third decimal place!)
Nothing to notice, but something to understand.
EDIT: With bombers;
1.3 x 1.2 x 1.1 x 0.6 x 1.15= 1.18404 or 118.40% (3 skilled Leader)
1.3 x 1.2 x 1.1 x 0.6 x 1.2 = 1.23552 or 123.50% (4 skilled Leader)
1.3 x 1.2 x 1.1 x 0.6 x 1.25 = 1.287 or 128.70% (5 skilled Leader)
now change the mission efficiency using 4 skilled Leader as base:
1.3x 1.15 x 1.1 x 0.6 x 1.2 = 1.18404 or 118.40% (same as 3 skilled Leader drop)
1.3 x 1.25 x 1.1 x 0.6 x 1.2= 1.287 (same as 5 skilled Leader increase)
Hmmm, I think it might have just been a rounding issue that I was seeing that might account for any variance, also while playing I'll be changing unit sizes and Leaders and missions between battles in which the units have gained experience and slightly changed the modifier that way, plus it is different for each air unit if they have performed more missions and have different experiences between them. I apologise for my confusion.