Both implant sets are hardly comparable as they function in starkly different setups. Crystals have no use in big fleet fights and are not used in these scenarios. Slaves work well in fleets, but are not used in big fleets, especially not HG or MG, as their cost is prohibitive to the high likelihood of losing your capsule. Slaves on small and medium fleet doctrines like Low sec Sacrileges and Absolutions only work because you cannot lose your capsule. Slaves on caps can be seen as problematic, but as this is only one small group of ships compared to, for instance, where Crystals are very effective (Solo Tengus, Shield Boosting Marauders, Sleipnirs, etc pp.), I don't really see a need to change Slaves.

Instead of reducing Slave effects, call for the introduction of a Shield Buffer set and Armor Repair set.

Slave sets are one of those thing which everyone knows about, and which have a major shifting influence on the sandbox, but which no one really seems to talk about much. Like many things which have been with us since the early days of eve, they have had an enormous effect on what doctrines and playstyles emerged. The ability for armor tanks to add more than 50% of their base HP in armor without a single concession on the fitting, in a way which scales absolutely with buffer mods, rigs and so on, is far more powerful in the modern eve era than the shield boost bonus that crystal sets give. That this extends to capitals, unlike crystal sets, just tips them further over the line.

I think it is time to tone them down.

Overall, I would like to see slave sets effectiveness dropped to approximately:

HG: 32%
MG: 26%
LG 18%

I would prefer to accomplish this by reducing primarily the per implant set bonus, and slightly reducing the HP bonus on slots 3-5, to 2.5%, 3% and 3.5% respectively.

Why:

1. Armor capitals are too tough relative to shield capitals, and this helps even that out.
2. Buffer bonuses are inherently more powerful than rep bonuses with modern fleet sizes for fleet combat.
3. Oversized plates and slave synergize unfortunately well on t3s, HACs and other cruisers with the space for large plates.
4. It makes finding a competitive shield doctrine easier, and I like shield doctrines.

To be completely fair to armor users everywhere, shield already gets a base hp buff from just the skills, so having a slave set isn't to unbalanced. It's just the natural outcome from armor boats with hp bonuses or high resists that they gain so much ehp. Slaves just have a tendency to be used on higher-level armor boats simply because they can sport such high natural resists, and it's easier to fit for them than shields are.

That said, I do support your idea with reducing their effectiveness. Cruiser and battlecruiser hulls reaching over a half a million ehp is absurd.

Actually it's 6x 80 million isk + 80.000LP from the Blood Raider LP store.

You are drastically off on how much a Slave set costs from the Sansha's LP store.