If you're happy with the performance/reliability of the 400W fogger, I'd say go ahead and grab another one. As much as I like Froggy's juice, it won't turn a 400W party fogger into a 1500W pea-souper. The Freezin' juice is designed to dissipate rather quickly; when run through a chiller, it's supposed to vanish before it has a chance to rise thus enhancing the low-lying effect. I recommend using the Swamp Juice and spend the rental money on making a couple of ice chest chillers. You can get old ice chests at yard sales, sometimes you find them at Curby's, too. Regarding the perf pipe with frozen bottles, here's a video of my Chauvet 800W and a 10 ft. perf drain pipe filled with bottles. I used Froggy's Swamp Juice.
http://s73.photobucket.com/albums/i231/Otaku1031/?action=view¤t=MOV06501.flv
http://s73.photobucket.com/albums/i231/Otaku1031/?action=view¤t=MOV06501.flv