Powders contain a lot of sodium carbonate, right? Don't think liquids do. I'm thinking that sc would maybe soften the water more giving it that slippery feel. Have seen articles stating that adding baking soda with liquids boost the cleaning power. I know the two are not the same. IME, borax does a much better job. I use it with both powders and liquids. If I used enough detergent to get a slippery feel, I'd never get it all rinsed out. The speedqueen is a great machine but does not rinse all that well.

Based on my experience and knowledge, liquids will not make the water feel as slippery as powders will. There should be some slick-ness, though. Borax will indeed enhance the cleaning power of liquids, and somewhat increase the softness of the water.