As a general rule, "Jack of all trades, master of none" is sound advice. These home-grade Best Buy or Staples specials are usually just plain garbage at everything. But trying to add on to PFSense and making that a WiFi router is also a fools errand.
Yes, you can use the ASUS as a wifi "Access Point" and the additions that the SG-2440 have are not internal, they are separate products from a company called Ubiquiti. PFSense doesn't control them, has nothing to do with them. Not to mention, you can get the UAP-AC-Lite for like $70. When you are adding in $549 for the SG-2440, then $70 is not much more and that is a TRUE Wireless Access Point. It doesn't do ANYTHING except be an access point. A fairly stellar one at that.
Just be warned, Ubiquiti's support stinks and sometimes more advanced features are half-baked or just completely broken and don't expect UBNT to be of any help. I was doing a fiber-backed wireless setup, and literally this new 10GB fiber switch couldn't auto-negotiate to 1GB. So even though you can go from switch to switch and manually set 1Gbps, the FiberPOE units they have coulnd't. I ended up having to get a non-unifi but still UBNT gigabit fiber switch. Talk about pain-in-the-rear. And by the time UBNT got back to me, I had it installed and working for days. And still had no real answer, just said "oh, well maybe thats rights, glad you figured it out". Zero clue. Same issues with their USG routers that always show as "Provisioning" in the status. Total bug and a half, but again they have no idea or don't want to admit it. I end up buying all my UBNT stuff from a company called Double Radius, and I use their RMA and support - although for high-level you are still SOL.
However, if you are just using wireless and want it to reach far, wide, fast and REALLY cheap, then UBNT is gold. If you want support or an RMA, throw it in the trash and buy something else. The next cheapest is Engenius Tech, but expect $300+ for a decent AP.
Wireless AC isn't great, as jahonix said. They have "Wave 2 AC" now, which solves some issues and now even "AD" wireless which apparently can't make it 10 feet. They keep raising the frequency, which is the wrong way. We need like ELF wifi. But the US Govt has a worldwide kibash on any usage of it because they use it for all of their super secret spy communications.
