For what its worth...My 2 Cents worth.
Improving a kit is a great way to learn and all the improver packs are OK. I have found that the very best way to improve whatever you are fermenting is a good understanding of sound practice so heres a couple of things that others may or may not have tried or understand fully.
1. Its OK to use cane Sugar......... Seriously.... It is. Most of our Major commercial breweries use it at the 20% level so that equates to about 1KG, which is in all the instructions. What they (The kit manufacturers) DONT tell you is that they use it as a KETTLE sugar. It is Never just added to a fermenter in the manner we have been told so the result is different. Boiling cane sugar in a high acid (Read Hop Acid) environment like a commercial brewing kettle will invert the sugar and split the Sucrose molecule up into its component sugars (Glucose and Fructose) so that the yeast doesn't have to. YOUR HB Beer tastes a "bit" homebrewy because the yeast Invertase taste is present. (Invertase splits the sugar in the fermenter)
2. This is the biggie........Rehydrate your dried yeast and preferably make a yeast starter. The more healthy, robust yeast you can throw in the better, this will fight the infections. Experiment with different yeast later.
3. Try to cold condition once fermentation is over. (you will then need to bulk prime)
4. Remember that Oxygen is good at the start (yeast need it and lots of it) BUT It is a brewkiller afterward. Yeast will grow volume during the Aerobic stage and then produce alcohol in the Anaerobic stage, once commenced you dont want to be going back.
5. Be gentle with metabisulphite. Its an Ok sanitiser but there are better about its main use in brewing is to displace oxygen when transferring to other containers or bottling.
There are other things you can do but ........slowly. Once you have some fundamentals sorted then move on to other ingredients.
Regards Dave