"There is no reason to go above 100 health, 30 speed and 1 Max count for Rockster because the benefits are tiny while the drawbacks are huge."
There is one of these at the Rockster Health upgrade from 875 to 900. The reward goes up by 1.73% but the damage needing to be done to kill it goes up by 2.86%. Since you earn Coins on kills, this means the return on the upgrade is negative. That is, this upgrade means kills (Coins) come less frequently but that is not compensated for by the size of the increase in the Reward.