Equihash (zcash) has more room for optimization in an ASIC than ethash.  The first round in equihash is dominated by blake hash speed, which is much faster in an ASIC than a GPU (and Bitmain already has blake miners).  The subsequent rounds can be significantly faster with a modest-sized multi-banked SRAM.  Something with 256 banks of 1KB should be at least 2x faster than a Rx 480, even if the ASIC is clocked below 1Ghz.