The aquagate seems to be fairly flexible when compared to the other premade kits like Evercool, Kingwin or Titan as they use proprietary tubing and connects. The aquagate can use standard tubing (although I am not aware of the tubing size - I think it might be 3/8 OD though) and employs barbs and quick connects.
Because of this, it - seemingly - could be customizable as far as varying from the orginal kits design or use. However, in looking at the hardware inside, it looks to use one 80mm rad and a single fan. The pump is not that powerful (compared to, say, the Swiftech 655) putting out about 150 gallons/hour. On idle it can make as much as an 11 degree difference (or so) and on load, 15 degrees - compared to a stock heatsink. This is using the fan setting in the kit at its highest setting (which, I read, is loud at 49db)
So you figure, you could probably get a better bran heatsink that could do considerably better than the stock. But, since you want to watercool, it just seem that adding anything else tot he kit than what it intended for would yield more resistance and worsening results. This doesn't bode well for what you hope to achieve though.
One 80mm rad and fan setup like this kit uses doesn't remove alot of heat already - adding more heat tot he loop might produce, quite literally, negligible cooling at best.
What is your budget for watercooling?