Ideally, the increment for each exercise should be a percentage of your 5RM weight. A typical increment is 5% of the 5RM weight. So, if 5% of your 5RM is 10kg, then using 10kg throughout the cycle ought to be fine.
Remember that the purpose of the increment is to raise the weight just enough such that the tension on the muscle exceeds the existing conditioning of the muscle, and thus stimulates more growth.
With that said, I don't really think manipulating the increments across the cycle is going to be all that effective. Using bigger increments during the lighter weights may speed up the conditioning of your muscles, making the heavier weights a bit less effective (a big maybe, here).
As for increasing the potency of the 5s, you could try incorporating some 5x5 techniques to increase the workload of your compound exercises a bit. Here's what I've been playing with: whatever weight you are scheduled to use for your 5-rep set, begin with a 5-rep set using 60% of the weight, then do the same with 70%, then again with 80%, then 90%, and then, finally, use your HST scheduled weight.
As a simple example, suppose on the third day of the 5s you are suppose to use 100 lbs for squats. Thus, you can perform the following sets:
set 1: 60 lbs
set 2: 70 lbs
set 3: 80 lbs
set 4: 90 lbs
set 5: 100 lbs
Not only with this provide you with more workload in the 5s, but you can also use it as a warmup for your HST set. But, please once you reach your 5RM weights, do not use this 5x5 approach for every single workout with ever single exercise. Only do it with the compound movements, such as squats, deads, bench, rows, chins, overhead press, and the like. And, once you reach your HST 5RM weights, you should modulate your volume--do the five sets on Monday (heavy day), then perhaps the first three sets on Wednesday (light day), and then the first four sets on Friday (moderate day).
Again, I must stress that this is an experimental approach that has worked far better for me than drop sets or negatives. But please proceed with caution, lest you burn out or get injured!