EMT has no (positive) impact on the network performance. Unfortunately, instead, it's a big grounded conductor being put very close to the cable's wires, which increases impedance deviations along the length of the cable. That, in turn, makes Structural Return Loss parameter of the cable worse.
I think the real reason that people a usually use EMT is because plenum cables are 3 times more expensive than regular compound PVC, and you could use PVC if the cable is run inside EMT in the plenum areas. However, over time, in many instances that requirement turned into a rule of requesting contractors to put BOTH EMT and plenum cables in at the same time for just in case.
The best way to look at the EMT issue would be from NEC (fire safety) stand point. I do not know any white paper or report that would deal with performance side of the problem.
Dmitri Abaimov, RCDD