it creates a very large unmetered air leak, the air flow the AFM is seeing is telling the ecu to inject fuel, but instead of it going into the engine it just escapes back into the atmosphere. This causes a tendency to bog when you shift to the next gear due to an over- rich condition.
I'd wait a few more weeks and save for the Real deal. Hks has a unique design. The valve will open under the very slightest PSI. I dont really like the sound (open air blow off) but it is tops when it comes to performing the job it is really meant for. Releasing pressure. Dont go the cheap way out with youre car Ace. I have the HKS, and it is worth every $.