hi guys iam geting a de-cat downpipe for my 03 sti they also sell heat wrap for exhaust, what is it and would i need it for my new downpipe.

 

It keeps the exhaust hot, keeping the exhaust gases hot, allowing them to escape faster, effectively adding performance however small the gain may be.
Also good for heat management under the engine bay, helping other parts run cooler. Its always good to have some, so you might as well.

 

It's one of those mods that has no downsides, as long as it's put on right.