There are many bonuses to hydrophobic down. For starters, most of the current methods actually boost the fill power of the down. Some down fills, once treated, can go from 800 to 850, 850 to 900, etc. So, it actually improves the down itself. Secondly, it opens up the ability to use duck down, which previously has been a lesser quality fill because...it stinks...literally. The new hydrophobic downs mitigate the oils that make duck down not so great. Down products of any kind, also need to be washed regularly. Dirt and body oil is the number one killer of down over the long haul and washing down has always come with some risk of damaging the loft. When Sea to Summit's engineers first started testing their Ultra Dry Down, they started washing it repeatedly to see how much degradation of loft they would observe. They eventually washed a bag so many times, they gave up. The nylon was pooping out long before the hydrophobic down. So, washing hydrophobic down is easier, less likely to ruin the fill, and thus makes for a bag which will ideally, last indefinitely.
The other advantages are important to note. Most hydrophobic downs will take 30% more water and exposure to water to fully wet. They will also dry 30% faster than non-treated down. But here's the real kicker. If you get your non-treated bag wet in the field, chances are high that it will never recover. Hydrophobic down can be dried in the field (without the dryer needed for non-treated down) with 100% recovery to the down loft.
As for the additional cost, according to most down purveyors, the additional cost to consumer has an industry average of less than 5%. Some noticed a bump in prices when hydrophobic down hit the market, but much of that had to do with world-wide increases in the base price of down. After 2008 and the global financial crash, simply producing the birds to make the feathers got rather expensive.
The StS Trek TK II sleeping bag includes a "mesh storage cell". I've read that it is always better to store the bag unrolled, compressed, stuffed, etc.. Would you recommend this? I have room to hang the bag, folded over once, on a clothes hanger in my closet if that will prolong its life and quality.