I'm probably the last person that would know anything about the handling characteristics of a Porsche, but just throwing it out there... perhaps the H&R springs are to blame?
Just going off my experience with "normal" cars, lowered springs tend to compensate for what they lack in height with added stiffness. This in turn ruins the dampening capability of the struts and causes the 'twitchy' handling which you mentioned.
To keep the ride "comfortable" with a lowered spring, it could also have a varied spring rate which means it has loosely packed coils at one end of the spring and tightly packed coils at the other so that at normal speeds only the 'softer' portion of the spring is being used, so you really have to lean on it to reach the stiffer, more "track focussed" part of the spring which in turn means more body roll.
Also, to a lesser extent you could be compromising the roll centre height of the car (kind of where the suspension pivots against the body of the car, haven't quite grasped this one), so while the actual centre of gravity may be lower, the roll centre would be higher (the roll centre would have stayed at the same height, but the actual body of the car is now lower so the roll centre is comparatively higher).
I know the effect on a "normal" car can be quite severe, so I can imagine on a car with such precise tolerances like a Porsche these effects could cause severe changes to the handling characteristics of the car.
Driver error could also be to blame. Not to say you are a bad driver but it takes more balls to drive the Porsche around the track quickly then it would to drive the GT-R. I'd say the majority of your time was lost because you were lifting off when you should have been pressing harder - probably because the springs were making you feel as though you were at the limit. The rear-engine layout of the Porsche means to add grip to the front you actually need to accelerate harder!
We actually learnt this the hard way trying to drift a TechArt Porsche GT2 - every time our driver tried to kick out the rear end the car just gripped more and tucked its nose in!
Watch this video from around the 0:30 mark and you can see him struggling to keep the tail out - and mind you the driver was drifting in massive arcs the entire width of the runway in a BMW M5 only minutes before.
Then again I could be totally wrong, but it's food for thought. My guess is that the Porsche is actually capable of being relatively on par with the GT-R.