Previous shield research was lacking so I gathered some new data. Damage data was gathered by measuring the shield HP bar in pixels on a screenshot after the hit. I chose to test against 2 attacks from both sides of the damage spectrum, lumber swing being one of the heaviest hits and scuttlebot bullets being one of the weakest hits. I assume most other attacks will fall between this range.
First set of data: Damage to shield taken at depth 19 from a Lumber (in mission 6-2 Beyond the Axes of Evil) on Elite.
(A Lumber swing deals elemental, normal and stun damage to shields).
A Grey Owlite Shield without any UVs at heat level 1 took 95.833% damage to the shield HP from a lumber swing
A Grey Owlite Shield with a Stun MAX UV at heat level 1 took 83.888% damage to the shield HP from a lumber swing
A Grey Owlite Shield with a Stun MAX UV at heat level 10 took 78.888% damage to the shield HP from a lumber swing
A Grey Owlite Shield with Stun MAX + Elemental MAX + Normal medium UVs at heat level 10 took 73.055% damage to the shield HP from a lumber swing
Second set of data: Damage to shield taken at depth 19 from a Scuttlebot (in mission 6-2 Beyond the Axes of Evil) on Elite.
(A Scuttlebot bullet deals pure elemental damage).
A Grey Owlite Shield without any UVs at heat level 1 took 23.055% damage to the shield HP from a scuttlebot bullet
A Grey Owlite Shield with a Stun MAX UV at heat level 1 took 23.055% damage to the shield HP from a scuttlebot bullet (no change since the attack doesn't inflict Stun)
A Grey Owlite Shield with a Stun MAX UV at heat level 10 took 20.833% damage to the shield HP from a scuttlebot bullet
A Grey Owlite Shield with Stun MAX + Elemental MAX + Normal medium UVs at heat level 10 took 16.111% damage to the shield HP from a scuttlebot bullet
Discussion:
1. Stun max added a good bit of extra protection against the lumber swing. The difference in damage taken was 12.5% (83.888/95.833).
2. Heating the Grey Owlite shield from 1 to 10 adds a nice bit of damage reduction. Against the lumber it took 6% less damage (78.888/83.888), against the scuttlebot it took 10% less damage (20.833/23.055).
3. The elemental max + normal med UVs reduced damage from the lumber swing by 7.5% (73.055/78.888). The elemental max reduced damage from the scuttlebot bullet by 22.5% (16.111/20.833)
4. The combined damage reduction from fully heating the shield and adding Stun MAX + Elemental Max + Normal medium UVs is 24% (73.055/95.833) against a heavy lumber swing and 30.2% (16.111/23.055) against a scuttlebot. Reminder that the reduction against the scuttlebot is exclusively caused by the elemental max UV and heating.
Since damage and defense interact by subtraction, these findings make sense. Defense increases will have a larger effect on the net damage coming from weaker attacks. The most spectacular result comes from the Elemental max UV against the weak scuttlebot, 22.5% damage reduction. In comparison: elemental max and normal med combined give a much smaller damage reduction versus a strong lumber swing. Heating provides a more balanced bonus to the shields defense, since it adds both normal and elemental defense. The result is still a 10% damage reduction against the pure elemental damage scuttlebot. Against the lumber it provides a 6% damage reduction. The Stun UV acts as defense on shields, but since only strong attacks carry stun, there are no small attacks to compare to. The reduction that Stun Max achieves against a lumber swing is a respectable 12.5%.
Conclusion:
Shields should be considered a regenerating health pool that will tank lots of hits over the course of a mission. Shield defense can be improved by heating and UVs. Combined these can provide over 30% damage reduction depending on monster attack. Status UVs can provide a large bonus but not all attacks inflict status. Both UV types (matching defense UVs and status UVs) are very useful to increase your shield's defense. Heating adds a balanced bonus to both shield defense types and seems comparable to 2 medium defense UVs.
Some players might be so good at dodging that they never tank a hit with their shield more than once in a row. In that case the shield would never break and so additional defense does not help them. However, it is plausible that in Shadow Lairs, 1 hit will break an unheated shield without UVs more frequently because monster attack is so high. In that case, UVs and heat might mean the difference between tanking a hit or having the shield break and status inflicted on you.
tl;dr: heat your shields and get UVs if you can afford it, it's definitely not a negligible bonus. Obviously though, heating and UVing weapons and armor takes priority over shields.
Have you seen Glacies's epic threads about shield protection research? I would not describe them as "lacking". Of course, more research is always nice.
My sense, from earlier studies and from yours, is that status UVs are generally more powerful than damage UVs. Do you agree? (I mean, against attacks that contain status. Against status-less attacks, status UVs are of course worthless. Just as damage UVs are worthless against damage-less attacks.)
Thanks for your findings.