Don't all of the turbo units get their pressurized oil from a gallery, and that oil is gravity drained back to the sump?
In that case the turbo oil is just bled off, so not much difference between the before/after the cooler issue.
Definitely needs an oil cooler, and keep in mind what Duck-Stew said about maybe two coolers.
Only on the ones that are mounted higher than the oil level in the sump. A turbo mounted under the swingarm (unless it is a sealed unit like the Airesearch Bill Johnson used) needs a scavenge pump to keep up the oil flow. Otherwise you can kill the turbo with coked oil in the bearing/bushing.