i'm pretty sure i remember reading that on linear springs like ours increasing pre-load definitely doesn't increase spring rate in any way. Rather it just changes the point at which the spring starts to react to load.
So to my mind increasing preload will actually make the car sit a little higher, cos there's less initial sag.
Everyone always wants to make the car nice n stiff thinking they're improving handling, and while this may work slightly on a smooth as silk track, on anything less perfect like a lot of tracks and definitely roads, it'll be way too stiff and just start skipping over every imperfection and understeering everywhere. Its all very well reducing roll, but not to the point you're sacrificing tyre/road contact and making it all skittish?
I know on B roads and bumpier roads around my area the cars i've had with standard shocks and softer springs would probably be much quicker A-B than when i've had coilovers on them. Depends how much you intend to use the car on track mate, but if it's gonna be used on the road a lot too i wouldn't go making it stiffer or messing with pre-load and things that most of us don't understand lol.
Car looks epic now, love the ph2 front end and bronze rims. Awesome.