That depends. A properly done retrofit with OEM projector components will be a significant improvement over the standard halogen headlights. Bear in mind that this isn't the case for all cars in general, just the WRX. There are some OEM halogen lights amongst other manufacturers that are far superior in terms of illumination to HID. I'm not sure about comparing the WRX OEM HID option to a true retrofit. I haven't seen any direct comparisons yet.
As for whether or not plug and play "HID kits" work legally in halogen projectors? The answer is still no. You can't escape the fact the bulb filaments between HID and halogen are different, and a projector focused to the objective length of the halogen filament will be skewed by an HID bulb. This is internal to the projector itself, and can't be fixed simply by tipping the lights down. Could you modify halogen projectors to properly focus an HID bulb? Probably, with enough time and effort, but a retrofit will all new components is likely a more time and cost-effective solution.