Koni top-adjusts will be OK, but you need to consider that requirements for the road vs requirements for the track are very different.
When considering a purchase of struts or coilovers you need to consider the cost of OEM struts and springs. Genuine OEM-spec items aren't cheap units, so you're going to need to step up to more than £300 to get better performance than the OEM kit (not just that feel better, because that's a very different kettle of fish).
On the road I found the Koni top-adjusts too short of travel, but I'm sure they would have been considerably superior to the OEM set up on a track.... but then you don't need the travel on a track that you need on the road.
I still think there's a very persuasive argument for OEM height struts on a Golf2 GTI, for a fast road car. This argument falls to pieces for a track, because you need far more control over a narrower band of movement. On that basis I'd argue for -40, but no more or you'll be compromising the geometry of the car.
