First, if I'm reading your post correctly, I think you're talking about the crossover pipe that goes from the driver's side exhaust manifold to the Y-fitting at the passenger's side manifold that sends the exhaust up the up-pipe to the turbo. The downpipe is actually the pipe that comes out of the turbo and connects to the exhaust pipe exiting the truck...that's a significant distinction in this case.
Reason being, if your thermocouple's mounted in the downpipe, anything over 1000* is way too hot...at 13 to 1400 degrees, you'd be looking at closer to 1500 to 1600 degrees in the cylinders, which will melt your pistons FAST
The general rule of thumb is that the turbo takes about 200 degrees off the exhaust temperature...couple that with heat losses in the crossover pipe and whatnot, and it can be even more. This is why most recommend getting the thermocouple as close to the cylinders as possible, ideally either in an exhaust manifold or in the crossover pipe as close as possible to where it connects to the manifold.
Second, even if your thermocouple's in the crossover pipe, 13 to 1400 degrees is still too hot....you really don't want to run an engine over 1200 to 1250*. While these engines have oil cooler jets spraying oil on the bottoms of the pistons, aluminum still melts at 1250*...so, really, at any temp higher than that, you're playing with fire...
