When u are shifting down the engine will rev higher and the clutch will just take the normal pounding it gets. The higher the rev is the higher the damage will be, but those are just wear and tear, don't worry too much about that. Plus the factory settings already counted for all the abuse and your engine should be fine since the max pressure it puts into is within the limits set by the factory, everything should be fine.
But your clutch is taking a pounding, the more mods you add tot he car the more pounding it takes. The factory clutch isn't the best thing ever, most the time its just something that would get you normal driving. If you are adding more power, change the clutch, and you will see how much damage the clutch has taken, plus it will feel a lot better, lol
About using your left foot to control the brake its used common in rally and auto cross because most the braking is down by sliding the car a little, with good control the car will not slow down as much as applying the brake. Tapping the brake pedal a little with left while going full on the gas with your right during a tune it will tighten up the line of approach giving a faster corner.
Now I'm tired, lol