It may well work great in low-insolation ME in low-temp apps without much tweaking, but the PEX could still melt if you're not paying attention. Have you tested the stagation temp of your collector? If it runs 200+ in stagnation tests you need to adjust the angle for less summertime gain.
For space heating you'll get a lot more bang for buck out of thermal air panels (lots of examples of those on Reysa's DIY site too), than with solar hydronic. Solar air is inherently lower temp/lower loss (== higher efficiency) than domestic hot water systems, but if you had say, radiant slabs or above-the-subfloor tubing systems such as WarmBoard(tm) the temp & efficiency of solar hydronic will be similar to solar air (at 5x the cost.) Serioiusly- getting $25/month's worth of electricity at typical NE rates out of a $1000 hydronic system like Gary's at 120F+ operation isn't very likely, whereas for the same money you could build 80-100 square feet of thermal air panel that would deliver as much space heating as you'd get out of ~150-200 gallons of oil burned in an 85% combustion-efficient boiler.
Unless your datalogger tests were done on a section of thin-walled copper or brass drainpipe your temp numbers drainwater heat recovery don't mean much, and you don't have corresponding flow rates to work from, etc. Natural Resources Canada has studied drainwater heat recovery extensively. See: http://www.builditsolar.com/Projects...terHtRecov.pdf I can point you to other web resources for their test methods & data if you're really interested. 50%+ average efficiency on a 4" x 48" 3" x 60" or longer is quite realisitic at typical Maine incoming water temps. If you're heating HW with electricity at NE prices it's fairly cost effective, if natural gas (at current prices) less so.
Also, high temperature is not a measure of efficiency or effectiveness on any of this- running systems at the lowest possible temperature that will actually deliver the heat to where it's wanted it key to keeping losses low, and efficiency high. This is true of everything from solar hot water to space heating systems. Running 140F water in a solar collector to maintain the temp in a 70F room when it's 20F outside means your collector is running at about 30% collection efficiency, but if you can still deliver the BTUs to the room with 90F water (which you probably can, if you have a slab-radiant) your efficiency will be around 50%- nearly twice as much heat per square foot of glazed area. For comfort reasons thermal air panels with active fans sometimes need to be run at 100F+ or the exit air has too much wind-chill to the nearby humans, but they don't need to run anywhere near 120F. Simple snap-disc thermosats on the heat exchanger panel that turns the fan on at 100-110F & off at 80-90F are often the cheapest & easiest way to go, and quite efficient.