I just fitted mine tonight and it was a pretty simple mod
Use the Sierra rails and you need to drill 2 new holes and elongate 2 others
On the drivers side i used the rear holes as a guide and had to redrill the front holes. It doesn't have a nutsert in the rails...just holes (GTi seats bolt to the side of the seat and not under...if that makes sense) so you will need a nut/bolt to make it work. I also fitted some washers. The original Sierra bolts are smaller than the GTi ones but if you have the GTi rails just use those bolts...you will need to find nuts to fit though.
On the passanger side i use the front hole as a guide instead of the rear (already has threaded hole for the bolt) and then elongate the hole on the rail and drill a new hole in the "seat" at the rear. You will need to cut the fabric of the seat which will expose a pre-drilled hole that is "out of centre" for the rail. You will need to drill a new hole above the existing one and then just bolt the sierra rails on. (Make sure you put some washers so the bolt doesn't slip through the hole)
I would strongly recommend using the GTi seats over the Autotechnica seats as i have both (one in each zook) and prefer the GTi seats....much more comfortable and fit much better
I've got 2 sets of Sierra seats (each set from each zook)....one in decent condition and the others are ripped. If you're interested just shoot me a PM