"I pay for a 150mbps connection but only get 30-50mbps"
So your saying wired to the powerline adapters you only see 50mbps.. But if wired to pfsense directly or with normal switch you see your full bandwidth? Or when connected to the modem you see full but pfsense is downstream of your modem via powerline? Can you draw up the constraints your working with.. Why exactly can you not run a wire from the modem to the rest of your network? What powerline adapters are you currently using.. They are not all created equal that is for sure..
"buy a new wireless USB"
Usb is just going to be crap.. I wouldn't even consider this as an option..
"use LAGG or some other combination of wireless and wired to combine the interfaces for speed"
Not sure where you get the idea that lagg will give you more speed? 1+1 = 1+1 it does not = 2 in lagg.. So while you could load share across the lag. A single client is not going to see any sort of speed increase.