I bought my 944 kit from BCM which included rebored and coated cylinders with the exchange of my old ones. I think it was around a $1000 for the pistons and cylinders. Before that I had just the stock displacement, high comp kit. There is a slight difference in power with the added 40 cc.
Also, its very important that you retard the timing a bit with high comp pistons. I put on about 25,000 miles and ended up burning both pistons during a track day before I went with the 944 kit. Detonation occures with the high compression pistons in the 2V motor cause the motor doesnt flow as well as the 4V ones. Retarding the timing reduces detonation and will prolong the life of the pistions.
Mike