I have the Campy Gran Sport components on my Austro Daimler Vent Noir including the brake calipers. When I bought it about 2 months ago they stopped just so so.
In examining the Campy pads I noticed they had flakes of metal in them and were well worn. I removed the metal flakes and sanded them down a bit to even out the wear. I also scrubbed down the rims to remove the black pad wear accumulation.
This improved the performance noticeably and I find them adequate. I don't find them quite as good in my experience as modern calipers such as Shimano RX100 and Ultegra that I have used on my other bikes but for my purposes I don't find their performance lacking.
I will probably upgrade the pads soon because even with the sanding they are fairly worn.
Safe Travels,
Paul Cunningham