The point is, compared to old sunfire you're suggesting +300hp, +25% bonus hp (which is +188hp on that item, and probably around +500 hp from all items combined [assume you can build 2k hp, should be easy with 750 from sunfire alone).
That's WAY too big a bonus.
Let's consider cinderhulk now as a top laner:
First, you give up a summoner. So are you teleport-smite or are you flash-smite? The former you're a gank target because you don't have flash, the latter you sacrifice a lot of map pressure for your team. You can't teleport bot lane for a gank in to dragon but your opponent can. You can't split push bottom late because you can't get to the baron fight on time, etc...
Let's assume you don't care about map pressure and only care about lane, so you go flash-smite (from the comment it sounded like you normally go flash-ignite and were willing to give up ignite). Let's even ignore the teleport advantage.
You build cinderhulk: you get 300 health and 25% bonus health and immolate
vs.
You build your suggested sunfire cape: you get 750hp and 25% bonus health and 45 armor and immolate.
Like seriously, even without the two passives the health upgrade alone makes it rival Randuin's as an armor item for less gold.
That sunfire buff makes it WAY too slot efficient. Even if it costs 3500g it's still a core buy because it gives so much for one slot. It's only 50hp short of Warmogs (before the passive, has MORE after) and Warmogs only gives hp (ignoring passive).
If you want bonus %hp on it, you need to sacrifice a lot of the hp and possibly some of the armor, and probably need to increase the cost still. Cinderhulk can get away with the %hp because it doesn't give any armor.