Because Braswell boards are not yet widely deployed, I suspect you will not find direct, authoritative guidance. However the Silvermont architecture has been around a while and Braswell is almost the same CPU architecture with a process shrink. Go nuts with Google searching for PFSense and J1800, for example, to get a sense of what to expect. I read some of the posts to suggest that a J1800 is generally good for 1 GbE WAN speeds so I would be comfortable with a Celeron N3000 (the really slow dual core you did not ask about) for your 0.15 GbE WAN connection. Iptables, however branded, is what PFSense is built around and it is a notoriously non CPU intensive kernel feature. It might not hurt to repost the question on a dedicated PFSense forum for greater certainty.
I do not like the motherboard you did not specify because it either features an onboard Realtek NIC or is ungodly expensive with onboard Intel NIC's and a power sucking BMC (Supermicro). Double check to verify that the onboard NIC is supported in PFSense as this has sometimes been a problem in the past. BSD and it's derivative distributions generally value Intel NIC's highly. Your unspecified motherboard may feature only a single PCIe slot. Consider a, perhaps used, Intel PRO/1000 PT Dual Port Server Adapter.
Do not get overexcited by TDP specifications. Intel shares a chassis thermal solution between several processors so they throw a bunch of different processors into an identical TDP rating. To date, I have not found a solid, apples to apples comparison of power requirements for different Braswell processors.
