we don't have numbers and mileage might vary. Normally the performance difference is recognizable by packet loss.
But the libpcap based approach does add quite some overhead, as libpcap provides a callback based interface per packet, might enforce additional copies and most important, libpcap requires the CGO interface adding a function calling overhead per packet. af_packet on the other hand is plain GO directly accessing the in shared memory (shared with kernel) + buffer sizes are somewhat tunable.
libpcap supports SOCK_PACKET and SOCK_RAW, but it depends on compile-time flags which one is available. Plus, libpcap tries to use SOCK_RAW/DGRAM, but might silently fallback to SOCK_PACKET reading package from socket.
Using af_packet you ensure SOCK_PACKET is not used, buffers are configurable + you remove some go to C and C to go calling overhead.