Sunfire is one of my favorite items in the game. I think the AOE magic damage it puts out is well worth the price, and I consider it to be a good mix of defensive stats while still being able to put out damage. It's well rounded for many situations, and is useful for both team fights and clearing.
I run heavy magic penetration on lots of champs when it comes to runes/masteries though.
In theory it sounds like Sunfire Cape would fill a good niche. An item for a beefy (but otherwise low dps) champion that otherwise lacks a constant source of damage? Sure, sounds like a plan.
However, in actuality the gains and the AoE radius on it are so low, it's right up there with old Tiamat in terms of overall inadequacy (ignoring that old Tiamat had good gold efficiency). It's still stuck in its old Season 1 design when they allowed players to stack it, and other similarly outdated items have either been removed or refined to up their potency by today's standards. Hydra's being very beneficial for the likes of Renekton / Lee Sin and so forth, Liandry's being tailor-made for junglers like Amumu / Sejuani and even Statikk Shyv being a more reliable source of increased damage / pushing power on a select few bruisers further expose the need for Riot to take another look at it. Maybe give it a late game upgrade path whilst simultaneously making it more manageable early on for the losing laner, or an active hook similar to the one
Boken imagined so that its overall damage output is more 'controlled' and has more reactionary play involved.
Then again, the high frequency of disengages / mobility also plays a huge part in it feeling lackluster.
Sunfire Cape is a very good item especially as a rush item.
No one rushes a Negatron just for that dude with the Sunfire Cape.
Aegis/Lockets aren't even commonly rushed these days. Who rushes Visage? Warwick maybe? Banshee is a pretty rare pick as well.
Like mentioned, it's a great mix of offense and defense for frontliners. Your other alternatives are Gauntlet (expensive, mana), Abyssal. No idea why you have to evaluate its damage post MR when it's applicable to every other damage source as well.
Sunfire Cape is nearly exactly like Renek's Lv1 ult, and somewhat along Rammus' ult in terms of damage.
Outright rushing Spirit Visage beyond a core component? Few, and that's match-up dependant anyways.
Who ends up buying it? Warwick, Yorick, Vladimir, Renekton, Susan, Trundle, Irelia, Jax, ... Pretty much any bruiser champion that's half-competent at brawling or self-sustaining. It's
the high-end MR item right now, as others here have also mentioned. Banshee's is on the situational side (Abyssal too) admittedly, but it's noticeably better after they revised it with the introduction of Spectre's Cowl.
Aegis / Locket is still bought virtually all the time, be it by the tanky melee jungler or the support. Not sure why you feel it's not common at this point in time.
On paper it sounds like a good item, but you can't tell me with a straight face that +5 armor and +70 health for a whopping 950 gold is a good defensive boost (and I already ranted about its offensive shortcomings). Finally, the idea behind evaluating its damage post-MR is simple: unlike other damage-oriented items or skills like Renekton's or Rammus' ult (which have a bigger radius by default), Sunfire's passive
doesn't scale positively.