Does enabling (or disabling) QoS on your router/modem effect computer security in any way?
Qos has in itself nothing to do with computer security, and is often related to VoIP and multimedia streaming.
One might need, or not need, such a “performance protocol” supposed to ensure that the needed bandwith is enough.
It is therefore not relevant to know if Qos is, or is not a security threat (and it does most certainly not participate to a better security), but it is enough to know that whatever uneeded service and protocol is, by definition, another security risk.
Note that, without even speaking of the modem router, and at least in XP, Qos RSVP service can be disabled if you wish (and use no software wanting it to run, e.g. Skype).