My disagreement schoppy, was that temps can absolutely be matched across a plate if properly sized. With my 40 plate at home, I checked it a couple weeks ago to have this discussion elsewhere and with the shower running, I had 168 coming off the plate into the tank. At the same moment, I had 168 coming into the plate from the boiler. Exact temp match. That's my residential example. I work commercially as a pipefitter and build hydronic systems every day, and I can assure you that there are thousands of commercially installed exchangers out there that match temp as well. Most are shell and tube design, but there are a handful of gasketed plates out there as well. As for the storage ordeal, that is a very common misconception. I fill my tank with 160-170 degree water, but it'll last a week in there without firing up the burner. To prove this point, I shut the gas valve off to my water heater last year and used a ton of hot water the day before I left for Aruba. I was gone for 6 days, and had enough hot water left to shower when I got home. I think having my plate below the WH allows for some thermosiphon, because the line is always hot.
RSI, I don't see how that would be of any benefit? First, it would eliminate hydraulic separation. Second, if the 15-58 dies the 26-99 will still pull the same water from the boiler right through the 15-58 while piped with closely spaced tees. What am I not seeing?