Just installed a Greddy turbo kit on my sons rebuilt LS motor. He wants more
power and I was thinking the quickest and least expensive way would be a new
larger downpipe. Are my toughts right and if so which size pipe would you
use. Thanks,

 

2.5" would be a huge increase, but only if the exhaust is at least 2.5"

If it has a stock exhaust, a bigger DP will not make any benefit.

 

2.5 or 3" open downpipe lol