Using @PeterKietzmann's l2 reflector, we've identified a couple of inefficiencies in gnrc_netdev2_ieee802154, mostly through unnecessary but expensive pktbuf alllocations:

• always calls gnrc_pktbuf_realloc_data() (fix in sys: net: gnrc: only reallocate if necessary #6294)
• iovec for pktsnip is allocated in pktbuf
• allocate whole pkt, allocate header, mark payload, throw away header is inefficient.