... Crit for 500 damage on 200 Armour... you'd take 20% of that as true damage, based on the OP's numbers... That's 100 true damage received...
With 20% LifeSteal, if you Crit 500, you'd also heal 100 health...

So either it's reflecting before damage reduction and unbelievably broken OP, mandatory for EVERY CHAMP in game to auto kill the AD with no issue.

Or...

It's reflecting damage AFTER damage reduction and two vamp scepters (or a BT) completely nullifies it. Making it essentially worthless.

Pick your poison, either way the change is flat out retarded. Sunfire you might be able to get away with (though I think it should just be a toggled AoE with a small scalar) but the thornmail change is borked.

If the crit's damage was 500 against 200 Armor, that means your ACTUAL damage was 1500, meaning you're taking 20% of THAT from Thornmail, which is 300 damage returned. Either that, or your crit should have been 500 but the Armor reduced it to ~165, meaning you only Lifesteal 33 from it. Thornmail pre-mitigation, Lifesteal post-mitigation.

Either way you do the math, you need 60% Lifesteal to negate that, and MR doesn't help anymore.

So Thornmail's reflected damage is pre-mitigation?...

By design, yes. Originally, Lifesteal and Spell Vamp were pre-mitigation, but Thornmail was useless, and for some reason LS/SV interacted oddly with shields, not giving healing off of shielded units. That was all changed in a patch quite a while ago so that LS/SV were mitigated somewhat by Armor/MR as well.

People are really forgetting how these two items are calculated, the returned damage from thornmail is after all calculations, it is 30% of what the wearer actually took, if an AD carry crits a tank for 500 damage they will take 30% of that, which is 150 damage, if the tank has 9 magic pen and 10% magic pen against a target with 30 MR they will have (0.9*30)-8.5=18.5, at 18.5MR they will receive 84.4% of that reflected damage, .844*150=126.6, now if they have even a single Bloodthirster, they will negate nearly all this damage, since 20% of 500 is 100, they would then only take approx. 26.6 damage.

This is in a best case scenario, excluding an Abyssal Scepter or other form of reduction, even then, an AD carry will often/may have a GA and/or MR/level runes which will drastically reduce this received damage, which ultimately make purchasing the item worthless when compared to the alternatives, the same can also been said for the Sunfire Cape, since they will likely be taking much less than 40 magic damage/sec, usually less than 30, which over a 10 second encounter is only 300 damage, taking into account life steal this is also mostly negated.

Also, if you take into account melee bruisers, you can throw it all out the window and stamp "completely worthless" on these two items for their unique effects since they will likely have over 100MR.

Executioner's Calling's passive is what needs to deal true damage!

Thornmail's pretty balanced as-is since most AD carries don't really get magic resist at all, and Sunfire Cape's damage output is fine for the same reason. The one problem is, Sunfire's other stats are way too low for the additional cost.

In terms of the Thornmail, 3 scale vests (a lower-tier item) give 135 armor at the cost of 2100 gold.
For 100 gold less and 35 armor, Thornmail gives a passive that usually counteracts life steal and makes characters like AD Yi almost entirely useless to the allied tank.

In terms of the Sunfire Cape, the upgrade's worth about as much as a Blasting Wand, and considering many tanks (Rammus, Malph, Taric, kind of Leona) have decent AP ratios, getting the AP would help their burst more than the extra 40 DPS to surrounding units if it didn't take up an additional two item slots. Since you're yet again looking at an item at a higher tier than Giant's Belt and Scale Vest, it should provide more stats than either (as is the case with most items) or provide a passive that's worth the upgrade (as is the case with Atma's.)

So Sunfire definitely needs an upgrade to its stats or damage aura, or at least a reduction in its cost. However, slapping true damage on it isn't the way to go, since you're not aiming to damage enemies who've built for high resistances.

BZZT! Incorrect.

Thornmail is BEFORE mitigation. It's based on how much damage you would have dealt if I had 0 Armor (which I don't, because Thornmail gives 100 and base Armor ranges from 61-90 at level 18). If Thornmail WAS based on how much you took, then it would ironically be mitigated by having more Armor. AFTER calculating how much damage that would be, the damage values go through mitigation (the damage of the attacker goes through the defender's Armor, and the defender's reflection goes through the attacker's MR).
Always good to look stuff up before saying something you don't know for sure.
So what you're saying is...

Bruisers can beat around Thornmail, an item designed to 1v1 DPS champions.

Which is not news. Bruisers are a cross between DPS, casters and tanks, more or less, relying equally on damage output, defense and spells. They might be more focused on one of those three attributes, but they use them all.
That's actually not correct either. Notice how most tanks are based on sticking to a target, getting into the fray, and disabling dangerous enemies. Now please, tell me, on which champion would 40 AP be equal to the DPS increase on Sunfire Cape, when you account for cooldowns, the limiting factor for damaging with AP.

Sunfire Cape is actually fine, all it needs is a less expensive combine cost. Some people would also like to see it put at 500 HP, 50 Armor, and I wouldn't argue against that, but all it needs is a lower combine cost.