Yes but that's only if you benefit from the lack of a second cable, it's not better.
I have learned from this thread to not assume anything even if it can annoy the other person ["have you done this, or this" xD]
By the way, the custom firmware on the zyxel supports 1500 on PPPoE or 1508 on IPoE if your router also does. In modem/router [not your case] the WAN interface requires a telnet "ifconfig ppp1.1 mtu 1500" [ifconfig beforehand to check if WAN interface is ppp1.1] on each established connection.