Here's a direct comparison of a Blouch 8cm^2 18g vs. a 10cm^2 hotside hta68. The cars are tuned virtually identically by Dom. The only possibly significant difference is the 18g has a full 3" turboback, while the hta68 car has a stock STi catback.
The 18g (thanks, Dr. Michael):
The hta68 (my car):