I don't know about other hobbyists in reef keeping, but I personally have a hard time keeping up with the costs of the reef system size I chose. I am always looking for a cheaper method, which will provide the necessary benefits. That is one reason why I like Randy's Do-it-Yourself supplements. With the increase in gas, electricity, taxes & the reduction in business profits (many are showing negative figures during this recession), I am with Mpthreer in trying to reduce costs.
That said, the cost of Instant Ocean (with needed supplementation using Randy's Do-it Yourself materials is probably cheaper in maintaining your parameters than any other option. There will be a happy medium between your salinity level and the cost of Do-it-yourself supplements comparing the IO salt mix costs and Randy's supplements costs.
What would be the advantage of running 1.027 vs 1.024 for instance?
Some invert do not fare well at low SG. Those you posted are really not that low for inverts as some reefs are 1.024. The issue with inverts starts more on the 1.023 or lower range. For fish it is not needed and some will say it lowers the fight for fish to maintaining their osmotic pressure regulation at lower SG levels. However, I just came back from the Texas area of Baffin Bay and Laguna Madre. In this area the salinity ranges from 40 -60 ppt and fish and inverts seem to have no issues and the fishing here is quite heavy. However, there are no reef fish here or corals. We use that 1.026 or 35 ppt as our guide line, as that is said to be std NSW, not that it can't be a little higher or lower, as NSW on real reefs do have the range you posted. And the Red Sea reefs are on the order of 40 ppt. It is more on the order of what seems to work best for your tank and its fauna and flora.
I tried 40PPT by accident for a couple of days. (( at the same time the temp dropped to 71*, normally 78-83* )). Ended up losing my Potter's angel pair, and that was it. (( Main pump failed while I was in the hospital --- unplanned --- the non fish person watching my tank was only told to top off. Since the main pump was off the sump level stayed about the same ))
My SPS and clams were fine. And for the most part my 2 Haddonis made it through okay, but one of them looked off. A month later it is now coming back to normal. The day I hooked up a new pump, and slowly brought the temp and salinity back to normal one of the Haddonis spawned.
Agreed with the above. I've nothing to add to that, so I'll just throw out some food for thought: tolerance to extremes in salinity vary hugely not only by type of organism, but certainly by species, and even among populations.
For instance: a study from a few years ago (the citation escapes me) looked at effects of hyposalinity on GBR corals. Normal salinity was 37 ppt around these corals (s.g., ~1.028). A drop to the lower 30's ppt caused low level bleaching in many corals, and killed some within a few days. A drop to the upper 20's ppt killed some within hours.
In contrast, a friend looked at the effects of hyposalinity stress on Siderastraea radians (which is a somewhat "weedy" species) collected from Florida Bay. Populations from areas that usually saw a salinity of ~35 ppt and rarely saw a salinity below 30 ppt did more-or-less ok down to a salinity of at least 20 ppt, but started to really have trouble and die in the teens. The same species from different populations that regularly saw hyposalinity stress (down to 15 ppt) did fine down to at least 15 ppt and didn't begin to fall apart until down around 10 ppt.
Not many organisms are that forgiving. As with the GBR corals above, salinities within the range people sometimes maintain in captivity are enough to really harm some animals. It's best to err on the side of caution, and shoot for a target that is likely to be acceptable by everything we might keep.