First off, the RX 480 will include 36 CUs. If we assume 64 stream processors to a CU – the GCN standard – then this brings us to 2304 SPs. AMD has not named the specific Polaris GPU being used here, but given the CU count I believe it’s reasonable to assume that this is a Polaris 10 SKU, as I’ve already seen Polaris 11 and it’s a very small chip better suited for notebooks.
AMD also revealed that the card would offer over 5 TFLOPs of compute performance. Given what we know about the CU count, this allows us to estimate the GPU clockspeed. This puts the lower bound of the GPU clockspeed at 1.08GHz and an upper bound (6 TFLOPs) at 1.3GHz, which would be in the range of 10-30% higher clocked than comparable Radeon 300 series cards.
In terms of raw numbers this puts the RX 480 just shy of the current Radeon R9 390. However it also doesn’t take into account the fact that one of the major focuses for Polaris will be in improving architectural efficiency. I would certainly expect that even at the lower end of clockspeed estimates, RX 480 could pull ahead of the R9 390, in which case we’re looking at a part that would deliver performance between the R9 390 and R9 390X, with final clockspeeds and architectural efficiency settling just how close to R9 390X the new card gets.