The current profiles only map single band data to each R, G and B channel.
You can modify them yourself, using band math if you need. You can save your formula for the profile for reusing it later.
Then, for each channel, in the Color Manipulation tab, you can select how raw data maps to actual 8bpp R/G/B value, by manipulating the transfer function. The default rendering uses a 95% rule (it cuts 2.5 % of the pixels on each side of the histogram).
You can export data to GeoTIFF from SNAP (File->Export menu). But currently this does not use compression, so the size of the output will explode compared to the input JP2-compressed rasters.