I have seen a ton of LS1 fbodies dynoed in the past decade. Average gains from an aftermarket lid versus the stocker are about 10 RWHP.

Don't buy into any of that 345 HP rating crap with the SLP lid either. The LS1 was not accurately rated HP wise to begin with.

The LS1 can be a nice looking engine bay when done up and you don't have to add a bunch of "weight" to make it look nice either.