not sure what you're asking about the downpipe, but I am in the process of ordering a full turbo kit from TMAturbo. A guy from the company priced me for a 3.8L piping kit at $1800. Seems pretty reasonable, considering the full street kit is $2300.
As for the down pipe, i assume you're not too experinced with turbo systems. A down pipe is a requirement for the kit, bov are not always included, esp in stock turbocharged vehicles.
A typical turbo system is as follows (correct me if i am wrong):
Exhaust manifold connects to the turbo, when running the exhaust pushes a fan (spool), which in turn pushes a mirror fan. The mirror fan pulls in air and redirects that air to the intake manifold (boost). the downpipe connects to the other end of the turbo and ends with the catalytic converter (if you have one).
Some drag applications do not really have downpipes but have drop pipes (or none at all). But if you want your car to run right as a street set-up a downpipe is a requirement.
Just check out this diagram below.
Its for a wrx but it is the same concept.