Late to the game here, my apologies... But here's my $.02
Hid's from the Retrofit Source are second to none, they produce a quality product and back it up (warranty). I have an HID kit from them I placed into NEW stock housings. Yes it can be done, two things are relevant, one is the filament on the HID lamp is positioned differently into the housing and if you measure the stock placement and subsequently mimic that with the HID's, the light thrown from the housing will be just the same, but more intense. Now, we all know the stock housings sux, so that is why many opt for the projectors as the projectors will put more light on the road where you need it.
Anyways, there was a write up on fixing the placement of the hid lamps correctly into the housing with stock headlights, search... If you can't find it let me know, I think I have some screen captures of it. And even then, I have my stock lights aimed a little lower than usual and I never get flashed... But they are bright, lol
that said... $280 is a steal, something I may consider for my GTP ; )