I am using a routed braid track with a 1/8 slot. We don't run t-jets. I have not noticed anything about performance with HO cars, granted, you would have to do a side by side comparison to see if there was any real handling issues. In other words, it works fine for HO, and we don't use anything but standard TOMY shoes and guide pins.
Marshaling is definitely easier with a 1/8 inch slot especially if the slot has a rounded edge.