When a difficulty change reaches the upper or lower bound (4x current), what is the implied impact on time per block?
The network doesn't "throttle" the blocks found. Rather, it simply doesn't change the difficulty by a factor of more than x4 in every adjustment (which is once every 2016 blocks). There is no limit on the rate of block generation.

Let's say you were in an equilibrium where a block is found every 10 minutes. Right after a difficulty adjustment, more power joins the network and the hashrate is x100 the original. The difficulty didn't change yet so now a block will be found every 6 seconds. This will go on for 2016 blocks (~3 hours) and then the difficulty will go up by x4. Now a block will be found every 24 seconds; after 2016 more blocks (~13 hours) the difficulty will go up again by x4. Then a block will be found every 96 seconds, for 53 hours; then a block will be found every 384 seconds, for 215 hours; then the hardcoded limit will not be active, the difficulty will go up by a factor of (600/384), and now a block will be found every 10 minutes.

In those times that the total hashrate is too high for the current difficulty, the level of forking will be as if the time constant was shorter. So forking will indeed be high if the total hashrate has just rapidly increased.