Aren't you growing in rockwool cubes or slabs? While a slightly lower res temp does hold more DO in it, which is a desirable trait, the thing that will most affect the temperature absorbtion rate is the overall temperature of your grow room. Rockwool holds a lot of water, and after even just 15-20 minutes, the ambient temperature of the air will raise the temperature of your growing medium anyway. You would have to flood your tables/trays, extremely frequently in order to keep the entire medium at 65* for any length of time. If you do that though, then you risk overwatering your plants, which will give you a host of other problems. IMO, the cost of the chiller, and the cost of running it, don't measure out in results unless you're doing aeroponics, or nft style setups.