That's not a bad idea - but I don't think it saves much to wait until mixing to have the additional audio tracks, because the lion's share of the work is in the conversions to audio, so you might as well have the multiple audio tracks present even during the recording process.
In other words, Sonar has to convert each instrument's midi-triggered sounds to analog, and that's what chews up the resources the most, even if it all gets routed to a single stereo pair of audio outputs.
By approaching it as I do, where the multiple audio outputs are set up right in the beginning of the recording process, I immediately get the benefits of controlling individual volumes, and even addition of low-resource effects, even while recording those instrument's midi tracks.
Sonar does not have to do much to place each instrument's audio data on the additional audio tracks that I have set up.
I think the above is a lot more flexible, powerful, and easier to work with.
Give it a shot and come to your own conclusions.
Please note that I have a computer with 32 GB of memory and multiple SATA III disk drives, with things split between the drives, so I never have to freeze during the tracking/recording process, but at the same time, as I noted above, I don't think Sonar has to work particularly harder to output the already converted data to multiple audio tracks, versus only 2 audio tracks, because the data being converted is the same either way.
I hope I have explained my approach well enough for you - again, please try it out and post back.
Perhaps make a complete copy of a project file within the same project folder, where the original project file has it set up your way, with all instruments from a Kontakt instance routed to a single stereo pair of audio outputs, and the copy of the project file set up to split all of the loaded instruments in the Kontakt instance into a stereo pair of audio outputs and then routed to corresponding multiple stereo output audio tracks.
I believe you will not see much difference in the work Sonar has to do for either of the two approaches.
Edited to add something - I also set up the buses and route the audio tracks as needed when I initially set it all up, so that I get quick access to volume for all drums, or even of a bus for just the toms within the overall drums bus, and similar setups for string sections, and horn sections, etc.
I have several sets of Track Templates set up for specific groups of Kontakt instruments, with all of the audio tracks and the outputs section routing assignments in Kontakt all set up, all going into a track folder, as well as the associated buses and the track routing assignments to those too, so it becomes SUPER quick and easy to add a strings section, or a brass section, or a general group of instruments.
I do this too for Battery, where I build custom kits for different kinds of standard kit sounds, and for additional percussive kit pieces (timpani, gong, hand clap, etc). Each kit piece gets routed to its own stereo output channels in Battery, and I also set up the associated audio tracks, buses, routing assignments, as well as a general relative volume starting point. All of this gets saved into a Track Template for that particular kit, giving me again SUPER quick and easy inclusion of any one of a number of REALLY nice drum kits, with all of the drum audio tracks and a single midi track set up in a track folder for Drums. (I sometimes include an additional midi track for the kit, to give me the ability to set up a more complex click track for a given section of a song, without disturbing the main drum midi track.
The combination of multi-timbral synths, such as Kontakt 5 and Battery 3 (or 4), their ability to route output to multiple audio channels, which can then be picked up by multiple audio tracks, placed into track folders, assigned to multiple buses (main drums, toms, cymbals, etc), all into Track Templates, is AMAZINGLY powerful in Sonar, and something I rely on heavily.
Track Templates are one of my FAVORITE features of Sonar, and as noted above, they can accomplish GIANT bunches of tasks with the simple inclusion of them into a project, with only a need to think through what you want a given track template to do, and then set it up into a single or into multiple track templates, as needed, for recall at any time in the future. GIANT time saver that gives consistent results.
Bob Bone