Im new to cox and I'm paying for 2 gig fiber service and getting 136mb to 158mb download speeds over wifi 40ft from the router. Is this normal for cox?

A 2.5 or 10 Gbps NIC is needed for more than 1 Gbps speed. If OP is paying for a 2 Gbps plan and has a 1 Gbps NIC, he's paying extra for speeds that can't possibly be obtained. But, with 158 Mbps current download, it's unlikely a faster NIC could get anywhere near 2 Gbps anyway.