first of all, there is no such thing as a 2" DSB. a DSB denotes a DEEP sand bed. 2" is not going to do it. i believe over 3"...even closer to 4" or more is required.
secondly, i agree with Tizzo, in that you can keep your nitrate down easily without a refugium, and i suggest you follow the proper husbandry steps to accomplish this, instead of adding components to offset the addition, or lack of depletion, of an abundance of nutrients.
thirdly, i like my refugiums. i do think they help with filtration more than stated. i have to harvest about 6" in the bottom of a 5 gallon bucket every 3 weeks or so and trade it in at the LFS for credit. i don't think this growth is just from light.
i have 2 set up. one is one a new tank with very little inhabitants. this one grows alot slower than the heavily populated tank. what's causing this accelerated growth in the other tank?