I was involved in the challenge and earned some BTC. I speedhacked the code in the mealtime at work straight after I saw the puzzle back in January 2015 and started it on some server remotely. The first version could do about 100,000 keys/s using a CPU. After some improvements I boosted the rate to 700,000 keys/s on a single computer. Later I ran it on many computers and checked million of keys/s. Luckly I could recycle the code later in the August 2015 puzzle at https://bitcointalk.org/index.php?topic=1144807.0
I had no time to learn how to code on a GPU.