Caffiene will give you a buzz/lift at the start of training (it is not long term, similar to sugar). It will raise your metabolism (so is great for using while weight cutting). It will not make you gas, what excess caffiene can do is dehydrate you. If you are dehydrated, then you gas easier. If you stay hydrated and have a caffiene drink before training you will be fine. The downs you may feel are probably related to the sugar in the carbonated drinks not the caffiene.
Caffiene is a perfromance enhancer. If it wasn't, it wouldn't be banned by the IOC. Elevated levels can offer enhanced performances.
Although my athletic endevaours are nowhere near someone like Elvis, I must humbly disagree. I stay very hydrated..probably close to 3 quarters of a gallon by the time class starts and if on a day I choose to boost myself with a caffiene drink, I will automaticly gas quicker due to my elevated heart rate.
And it's not the sugar because I get "lo carb monster" with a total of 6 grams sugar or "sugar free no fear".
Some peoples sensitivty lies in different ranges..but for the majority I would have to take a wild stab at caffiene hurting one's cardio.