Since we're arriving at the OSB era, OSB RW can be used to bypass bosses weak phase which make the battle become significantly easier in many fights.
Because OSB RW has a potential to hit the damage cap when being used properly, even small percentage boost could noticeably affect the total damage. Therefore, we're looking at this in details.
Total damage from SB is the multiplication of:
[Offensive Stats Multiplier]
[SB Multiplier]
[Row condition]
[Damage Type Boost from RM and RD]
[Elemental Boost from relics and element attached]
[Elemental Boost from RM and RD]
[School Boost from RM and RD]
[Enemy's weakness]
[Weakness boost from RM and RD]
[Offensive Stats Multiplier]
For physical attack, it's a lower value between ATK1.8 / DEF0.5 and 4000 * ATK0.5 / DEF0.5
For magical attack, it's a lower value between MAG(MND)1.65 / RES0.5 and 3000*MAG(MND)0.5 / RES0.5
In case that you cast SB natively, ATK/MAG used in the formulas will be the total multiplication of [ATK/MAG], [Buffs], [Debuffs] and [percentage boost from RM] of the caster.
However, when casting RW, the total ATK/MAG used in the formulas will be replace with the RW's. This means any stats boost from buffs and RM will not affect RW damage.
[Row condition]
You get half damage output, if you use melee SB with melee weapon in the back-row.
In case that you cast SB natively, your character can equip range weapon to unleash full potential of melee SB in the back-row.
However, when casting RW, your weapon condition change according to the RW. This means you still get half damage when casting melee RW in the back-row, even though you equip your character with a range weapon.
[Damage Type Boost from RM and RD]
[Damage Type Boost from RM] is referring to "Deal adj. more (-physical, black Magic, white magic, summon magic) damage (when equipment is equipped)".
[Damage Type Boost from RD] is referring to "+ x% (Equipmentphysical, black Magic, summoning) damage".
Damage type boost from RM and RD are stacking additively and affect RW damage, if the conditions are met. The SB must deal the same damage type and caster equipment (not RW equipment) must be the same as stated in the RM/RD description.
[Elemental Boost from relics and element attached]
Elemental boost from relics and element attached are stacking additively and increases SB damage dealt by 20% and 80% respectively.
However, when casting RW, both elemental boost from your relics and element attached are replaced by RW's elemental boost relics. Therefore, elemental boost from relics and element attached buff of the caster doesn't affect RW damage.
[Elemental Boost from RM and RD]
[Elemental Boost from RM] is referring to "Deal adj. more elemental damage".
[Elemental Boost from RD] is referring to "+ x% elemental damage".
Elemental boost from RM and RD are stacking additively and affect RW damage, if the conditions are met. The SB must deal the same Elemental damage as stated in the description (care for multi-elements SB).
[School Boost from RM and RD]
[School Boost from RM] is referring to "Deal adj. more damage with school abilities".
[School Boost from RD] is referring to "+ x% school ability damage".
School boost from RM and RD are stacking additively and affect RW damage, if the conditions are met. Most SBs are in combat school which cannot be boosted. However, there is an exception (Zidane's OSB), so school boost is still worth noticing. The SB must be in the same school as stated in the description.
[Weakness boost from RM and RD]
[Weakness Boost from RM] is referring to "Deal adj. more damage when attacking with an enemy's elemental weakness".
[Weakness Boost from RD] is referring to "+ x% Damage vs. Weakness".
Weakness boost from RM and RD are stacking additively and affect RW damage, when hitting enemies' weakness.
Optimization
Since RM and RD boosts of the same catagory are stacking additively, while RM and RD boosts of the different catagories are stacking multiplicatively. You should choose the RM according to the boost that's not appear in his/her record dive.
For example.
Tyro who get +12% black magic damage and 9% damage vs weakness attemp to cast Explosion XXXII (Maria OSB: BLK / earth, NE) on enemy with earth weakness.
Equipping [+30% black magic damage when equipping staff] (Shantotto's RM3); total multiplier from boost (type * element * weakness) = 142% * 100% * 109% = 154.78%
Equipping [+30% earth damage] (Tifa's RM3); total multiplier from boost (type * element * weakness) = 112% * 130% * 109% = 158.704%
Equipping [+30% damage against weakness] (Tyro's RM3); total multiplier from boost (type * element * weakness) = 112% * 100% * 139% = 155.68%
Since Tyro doesn't have an elemental boost in his record dive, equipping elemental boost RM give Tyro the best damage output when casting RW.
summary
Caster's offensive stat, buffs, elemental boost from relics, and attached element don't affect RW damage.
Caster row affect RW damage. If you're using melee RW, cast it with front row character.
Caster RM and RD affect RW damage, if the conditions are met. relic conditions are referring to the caster's (not RW's).
Maximize RW potential by choosing RM according to caster record dive.