(Sorry to sound snotty, but would it be okay for you to use words instead of numbers? Much easier to read! ^_^
As for your horse.. drugs are bad unless they're painkillers in my opinion. I think that if you REALLY want to show, you should do so on a different horse. Remember, just like some people, some horses do not like to be thrown into a brand new atmosphere with new horses and new people and all that exited buzzing around nature of people at shows. That is acceptable in my mind, that some horses just do not like showgrounds. Remember that most horses don't see a great deal of point in shows, so theres no reason to make them do something like that. While you might find it fun, do you really think that the chance of a ribbon is worth sedating your horse? Sedatives are VERY frowned upon in the horse world these days, and if your goal with showing is big enough to make you keep on showing this horse, keep in mind that in upper levels drug tests are done. I belive Cian O'Connor was actually taken down fromt he Olympics because he sedated one of his horses or sopmething along those lines (might have been two horses).
Sedative slow down reaction time, so that's going to effect your performance. If your jumping or doing cross country, this can be VERY dangerous.
However, apart from shows, doing more desensitizing and familiarizing excersises can and probably will help your horse. Be creative and introduce him to bright hings, loud things, things that move easily. When on trail rides, go with whoever owns his pasture buddies so he'll have familiarity with him for the first few rides, and then bring a new horse along, until you have him going with horses he normally doesn't associate with.
But again, drugs=bad (Imagine if you were going to perform at a sporting event and you had huge bouts of nerves, and somebody just stabbed a tranquilizer into you. Not fun. Horses who get tranquilized probably associate shows with bad feelings)