If you can find a way to mark the packets you want then you can shape them. However the option you have selected is not what you want (http://www.frozentux.net/iptables-tutorial/chunkyhtml/x2702.html#TCPMSSMATCH). I’d suggest to use this match instead http://www.frozentux.net/iptables-tutorial/chunkyhtml/x2702.html#LENGTHMATCH