I dont know what excatly is limiting them, but temps doesnt seem to be a big part of it, delidders getting 20 degrees off still only see another ~200mhz headroom.
take this guy
http://forums.anandtech.com/showthread.php?t=2324197
He is running close to 100 deg. celcius and still stable at 4.7, the oc-ability of a batch is something else than just how cool it runs.
First of all, this is off-topic and not really worth discussing. But since you brought it up, de-lidding has it's own sets of problems, I can assure you that it is extremely easy to ruin your CPU by doing so: It's even riskier with the Haswell chip because of the integrated VR, so there is double the potential of causing damage especially if you're using the razor method. Using the Vice method is fairly safe but not everyone wants to spend 60$ on equipment to do that - just for 100 more mhz. Aside from this, IVB/Haswell both can run at 85C completely safe - there is *nothing wrong* with that temperature. People just don't expect it when coming from SB, but the temperature tolerance of IVB/Haswell is higher than that of sandy bridge.
The problem with Haswell, partially, is that everyone is judging load temperatures by prime 95. A little known fact regarding Haswell is that certain AVX instruction loads will cause the voltage to go higher by .1V, so if you have 1.25V set in the BIOS - prime 95 will cause the CPU to run at 1.35V. Obviously this has detrimental effects on the CPU temperature. And none of these temps will ever, ever occur outside of anything but prime 95 or intel burn test.
Anyway, long story short, the situation in terms of overclocking between IVB and Haswell is not completely different. I'm not arguing de-lidding, or any of that nonsense, what you're telling me is completely off topic. The point i'm getting at is that opting for IVB over Haswell for a NEW BUILD is completely stupid. IMHO. Haswell/Z87 is far better overall, and the Z87 platform is far, far better than Z77 was. That's my main point - i'm not here to discuss de-lidding and all of that nonsense. If you want to do that and are okay with the risk, go for it. This thread is about choosing Ivb over haswell - which is a nutty move for a new system.