I did buy and mount the SI Deck Lid on a LX 4dr Sedan... It was not easy since as you might have mentioned it doesn't come with template and/or mounting hardware, because it is a Replacement Part for SI models. It is much cheaper than buying the normal Deck Lid, although it is a bit more complicated to install.
If you want to do a close-to-factory install, it would be nice to buy the SI wiring harness ($15-$20)so that you can use the factory plug instead of trying to have it hardwired. For the drilling part you should measure twice or 3 times to make sure that you have accurately mark it. The SI spoiler uses only 2 bolts and 3 or 4 "plugs". The "plugs" are actually a bit cheap and you would need to be accurate on the drill holes to make them snap perfectly. Instead I changed the clips for 4 screws that snap in the spoiler and bolted them with lock nuts from underneath. The wiring harness can be found on the parts/accessories online store looking for the SI (I think it is wiring harness #5).
I would also suggest to try and look for the Heavy-Duty Trunk spring, since with the added weight it can drop hard when closing. The normal Deck-lid comes with a heavy-duty trunk spring, I wasn't able to find a part-number for it, but instead I figured that if this spoiler comes factory standard for the SI, the SI trunk springs might be the heavy-duty ones. So I bought the SI trunk spring ($20-$25). I haven't tried it yet... I'm waiting on the trunk spring tool so that I can remove and install. I'll try to post pictures as soon as I can for futures do-it-yourselfers...