Electronic – Shielded Twisted Pair noise immunity

Well, I gotta rush but https://www.dataforth.com/catalog/pdf/an507.pdf answers your question qualitatively

Cable shielding is used primarily to minimize or eliminate capacitively coupled interference from electric fields. When properly implemented, it can also be used to minimize inductive coupling from magnetic fields. Shielding is only effective against electric fields if it provides a low impedance path to ground. A floating shield provides no protection against interference. Grounding of shields can be a controversial subject because there are several ways to do it. The correct place to connect an electrostatic shield is at the reference potential of the circuitry contained within the shield. This point will vary depending upon whether the source and receiver are both grounded or whether one or the other is floating.

Quantitatively, i.e. how much worse/better... hopefully someone else will answer.

This may be reasonable or not, depending on the environment.

First, you should have reasons to suspect EMI may be a problem, otherwise I wouldn't bother.

Second, you should understand where the source of EMI noise is. If the source is near the device (or the device itself), grounding a back plane via the cable sounds like a good thing to do, provided the device is hand-held and cannot have its own proper grounding.

On the other hand, if the device is operated in a relatively calm (EMI-vise) environment, whereas the cable goes e.g. through a plant with heavy machinery, your ground connection will do more harm than good. The cable shield will effectively act as antenna, picking up all the noise from the plant and delivering it to your device, which would be much better off without it. In this case, you should still ground the cable shield, but not connect it to the device.