I agree with the lighting person to do or at least help you with your wishes. Not only will they be able to figure out and simplify your design intent than hopefully make it come true, but they will be able to add to it in hopefully making it better yet and make it electrically safe - given they are experienced with their profession which is also a grey area.
On the subject of PAR 64 fixtures replacing PAR 56 fixtures, yes there is an overloading your "cube" issue to look into, also that you don't need to change fixtures to up the output and wattage in that there are halogen PAR 56 lamps on the market which would save you money if your only goal is to up the wattage. I don't expect that even if you have the power available, a 1,000w lamp would be very useful for other than a rock concert sized production - too bright in a club. In other words, the PAR 56, re-lamped to a 500w would no doubt serve fine given it is UL listed for that wattage - another thing for the lighting tech to verify.
Otherwise if you can't get a electrician or lighting tech - stage or entertainment to help and at least advise, hopefully given your wish for a 500w lamp, others will be able to answer your power cube question. Remember that a 500w lamp is now going to eat up 4.1666 amps of current as opposed to 2.5 amps of the 300w lamp. This is very important to look at in powering up your system, cords feeding it and the capacity of the dimmers or switches controlling it all. Chances are, yes you will have more light, but might not be able to power up as many fixtures per circuit. An option might be to use the higher wattage lamps for the more saturated colored beams of light and the lesser wattage ones for the more amber or less saturated beams. This will help in overall power consumption, but not much on an individual control circuit basis if it's limited in power.