Stock projectors housing = waste. H7 bulbs = waste. If you want better light output, you NEED OEM HID bulbs (D2S) and you NEED OEM HID projectors. The lens alone will not help with actual light output, and just throwing an HID kit in will more than likely reduce the light output compared to the stock halogen bulbs. The lens will help the cutoff some, but your halogen projectors will still produce a good amount of glare, have a very narrow beam, and well, suck.
Since I haven't opened the headlights up yet, I have no clue how difficult/easy it's going to be to swap the projectors. The projectors I have are from an 07 STI. I was told that Subaru didn't change the projector from 07 to 08 for the STI. So more than likely, you'd be able to swap them out rather easily. If you're serious about actually making an improvement, you need to go with full OEM HID equipment.
This kind of modification isn't for the kind of person who gets an 8000k HID kit off eBay for $50 and thinks it actually makes an improvement.