As far as I know, LoS increases any bonus that applies to the champion stats regardless of how that bonus is given. For example, a perception set will give 40*1.15 = 46 accuracy, and 5% * 1.15 = 5.75% speed, since both accuracy and speed are champion base stats.

What LoS will not do is improve other effects from sets, such as the amount of lifesteal you get, the ignore defense percent from the cruel set, or the chance to apply poison from the toxic set.

Thats correct. I think this mastery is best reserved for people like Arbiter or Madame Serris, who you will typically gear up in triple speed or triple perception. If you only have one 2-piece set that gives a stat bonus, you'll probably find more value in taking another mastery instead of wasting it on LoS to gain 6 aaccuracy for example.

I don't think that is quite correct, Swiftblade. While you're right that people like Arbiter or Serris may benefit more from it than others - the reality is that the other choices at that level are pretty much garbage, and more importantly, most champs going that far into the blue tree are doing so for Master Hexer. That means you're obliged to take one of LoS, Evil Eye, or Cycle of Magic. The latter is simply garbage, so if TM manipulation isn't relevant (because the target is immune, for example), then despite LoS giving a relatively poor RoI, it still ends up being the choice for most - especially if you aren't going to be able to take both Sniper and Master Hexer (which puts Cycle of Magic out of the picture).

Increases the Base Set Stat Bonuses of all Artifact Sets THAT INCREASE BASE STATS BY 15%. This increase is multiplicative, not additive. (...)

That means, for example, that a Cruel Set that includes ATK +15% would be included. But it would do nothing for a Perception Set, which is ACC +40 and SPD +5%. Also, it would do nothing for Speed sets since they increase speed by 12%???!!!

First of all, it's just not true, as you can test easily by giving an accuracy set or a speed set to a champ with lore of steel. The set bonus of accuracy and speed IS increased.

Second, what you write is absolute nonsense. The way you interprete the "by 15%" doesn't make any sense, because with your interpretation, the whole description of lore of steel would not include by wich ammount the base stats are increased. So obviously it has to be read as "Increases the Base Set Stat Bonuses by15%. Works for all Artifact Sets that increase base stats."