I tried your suggestion of opening multiple Sonik Synths and loading one sound in each...it works well. Since I am more of a songwriter and guitarist than a synth head, this is a quick and easy way to add Sonik Synth textures without going too deep into programming a multi-timbral patch. I think you are probably correct in the assumption that single sounds are easier on cpu load. Plus, I can control each sound's volume easily this way.
However, both techniques are great to know and I plan on using them both! Note that in Logic 303 Tips 'n Tricks 2 (September) Steve H. offers a good explanation of mapping CC11 (expression) to give a multitimbral synth separate volume controls - very cool.
Many thanks to you and Rounik for expanding my knowledge of Logic and Sonik Synth 2!