Yes, such a problem does exist. The kangaroos still find the DP, because of their huge amounts, but the paths are too short and the first DP found by single kangaroo is always much worse than the subsequent ones.
Correct. So when you start app with DP=32 you can think that every point in DB represents 2^32 points. And DB grows as expected, but in fact, first several months DB won't work as expected because DPs represent much less points. It's the first overhead. And, of course, there is second overhead with collision, when kangs collided but it will take a few months to get to DP.
And both these overheads don't depend on the number of GPUs that you use, so even if you use thousands of GPUs probably you will lose like a year
