So we have somewhere around ~1270 stock and ~1350 overclock, and we speculate that AMD TDP limit might be so tight that even a 6% overclock might hit the power ceiling?! If that would be the case, prepare for the card to throttle at stock turbo clocks, because power variation alone can lead to short term spikes like this.
Why would you do such a poor job and potentially gimp your own product (launch) when you can set up more accurate temperature limits along a configurable TDP, and let the AIBs prove their worth by providing added value where they're good at? (stronger quiet cooling) The cooler itself makes for the best hardware limit!
Because they've learnt not to make a crap reference cooler since the 290/X debacle.
I've seen the RX 480 benched live, low 70s temps and quiet. The cooler is in no way the limiting factor, if it can't OC well, ie 1.4ghz tops, on the reference card but runs 1.5ghz stock on custom cards, it means it's artificially gimped. That's all there is to it.
They did this before, Nano had a very tight TDP regulation via the bios. It will throttle any OC unless you manually up the power limit which goes to 150%. Now, AMD could simply pull a fast one, and remove that option or limit it to 110% (NV's cards have 107 to 120% TDP settings). If they enforce a 110% TDP limit, then yes, as soon as you OC, it will throttle and so you can't get high clocks.
Now, ofc, the rumor could be full of BS. It could well be the reference card OC just fine with +vcore +power limit. Or it could be that Polaris is a crap over-clocker.
Since we're speculating, I'm basing it on the source with the claims reference cards are limited in clocks while custom cards come with 1.5ghz.