T-304 won't shield against low frequency magnetic field. It might protect against it if you build Faraday Cage but even then it is very limited. I'm no expert on Faraday Cage but as far as I remember it is used mostly as electrostatic shield. It works against magnetic field but poorly at lower frequencies.
We don't even know what is the nature of the noise pickup. If switching power supply has fast transients (high frequency) even non-magnetic shield would help cutting on capacitive coupling and creating losses (eddy currents) with magnetic field. I would stay on the course, since you already ordered shields. It will give you shielding at high frequencies allowing to isolate the problem.
I absolutely agree with Al's assessment on the Cat6 improvement. Jitter is not only function of the signal noise (not likely since it is buffered) but also system noise (receiver threshold noise) that is influenced by noise injected by current induced in the cable that finds return to ground thru the system. Ethernet uses differential signaling that cuts on common mode noise but it still couples high frequencies thru input capacitance to signal ground. Currents traveling on the signal ground are system noise. Judging by improvements you experienced your noise pickup might be of high frequency and your non-magnetic shields will work.
We don't even know what is the nature of the noise pickup. If switching power supply has fast transients (high frequency) even non-magnetic shield would help cutting on capacitive coupling and creating losses (eddy currents) with magnetic field. I would stay on the course, since you already ordered shields. It will give you shielding at high frequencies allowing to isolate the problem.
I absolutely agree with Al's assessment on the Cat6 improvement. Jitter is not only function of the signal noise (not likely since it is buffered) but also system noise (receiver threshold noise) that is influenced by noise injected by current induced in the cable that finds return to ground thru the system. Ethernet uses differential signaling that cuts on common mode noise but it still couples high frequencies thru input capacitance to signal ground. Currents traveling on the signal ground are system noise. Judging by improvements you experienced your noise pickup might be of high frequency and your non-magnetic shields will work.