You're not going to get much more instant horsepower out of the IC setup without having a refrigeration unit working. The drag of the compressor would override the gains.
Using a larger reservoir will give you a little more horsepower due to a larger volume of coolant but the biggest improvement is that it will take longer to heat up more coolant. That means you've got more horsepower for a longer period of time.
These guys are chasing their goal but they're simply over thinking and trying too hard.