"How does SI-1 compare with other similar products?"
I'm not sure there is a similar product. SI-1 has an ammonia smell and is very oily. It doesn't evaporate readily so I’d think low concentrations are probably best.
I soaked a piston in it once and it did dissolve the carbon, but that was the straight stuff. FYI, the carbon deposits were on the top of the piston, the ring pack looked new.
In another case, one of my Honda’s with over 200k miles has been given the SI-1 treatment for the last couple of years. Looking down the spark plug holes you see mostly aluminum. The piston tops have very little carbon on them.