So lets say you can improve something by 10% over all 5 tiers.
If you do all 5 tiers does everyone get the same 10% after applying all 5 tiers such that each tier grants a random amount toward that total?
So I might have 6% by tier 3 whereas someone else has 7% by tier 3 but by tier 5 we both have 10%.
OR, is it possible that we both do all 5 tiers and one ends up with 9% and the other with 10%?
Also, can you mix and match (or get all) effects or do you only pick one thing to upgrade to tier 5?
You can only apply one effect to a given module (plus 1 experimental).
Tiers below the highest rank you take it to have NO affect on the end result, only the highest grade that you apply. Ergo, stop rolling a grade once the next grade up unlocks, it is a waste of material. You do have to start at grade 1 for each module and work it up though.
When gathering materials, general rule of thumb that is working for me is to plan to need the following number of rolls:
Grade 1: 1
Grade 2: 2
Grade 3: 3
Grade 4: 3 to 4
Grade 5: ~7 (varies more than the others though if you are looking for absolute max)
If the engineer is NOT yet at full reputation/rank (5), plan for needing double the number of Grade 1, 2 and 3 rolls for the first module you do there.
The effect of the engineering is the same for every player, assuming they take it to the maximum effect of the maximum allowed grade (i.e. there's no real variable other than where you choose to stop rolling the max grade if you do not max it out).