[Plums]

I agree that for many Cobra drivers, these 1 5/8 pipes will do a marvelous job. For an untuned LS1, these pipes will be a great choice.

As soon as you get a decent MAFLess tune, a step to the next size would be ideal. Why? Most decent tuners can tune the "bottom-end" with larger pipes to equal, if not exceed the smaller pipes torque gain, with better top-end performance too...
Chasing maximum HP is not what it's all about. Why not get a few extra HP up top, while still maintaining the same bottom-end performance????

All of the above relates to stock, unopened LS1's. As soon as you put an aftermarket cam in an LS1, these smaller pipes will be left behind for top-end performance.
These smaller pipes will still even work on over 400 cube LS1's, they will just strangle it somewhat...

NASSTY has absolutely and positively hit the nail on the head!!! The whole system needs to be matched. There is one problem with Cobra's, people try to make them look original by using the 2 inch sidepipe bends and the large 3 1/2 inch collectors. 1 5/8 primary pipes that step-up to 2 inch pipes then a 3 1/2 inch collector will absolutely kill performance. The laminar flow that Ian mentioned earlier will be totally lost. Even a 1 3/4 pipe step-up to a 2 inch is too much and will hurt torque.
If you wanted to run these 1 5/8 pipes, I would suggest running 1 3/4 side pipe bends....