According to AMD's marketing team, the R9 Nano offers twice the performance density and twice the performance per Watt as the Radeon R9 290X. The R9 Nano has a single 8-pin PCIe power connector and a 175W TDP. The card has a single-fan cooling solution and the performance should be around R9 290X level.
This ultra-short model is reportedly based on a full Fiji GPU with 4096 stream processors, 256 texture units, 64 ROPs and a 4096-bit memory bus with 4GB HBM memory clocked at 1GHz effective, resulting in 512GB/s memory bandwidth.
Details about the core clockspeed are still unknown but rumors point to somewhere in the 830-900MHz range. The R9 Nano is expected to be revealed in August, pricing is still a mystery.