Is there any way to determine the usable range of Sonar Producer 8 soft synth patches?

I don't know of any complete database for Cakewalk synth ranges.  When faced with an unfamiliar preset, I'll often shift to the high/low octaves on my MIDI controllers to test the limits.  That's also a preset programming technique to see how your creation might perform when it's being utilized by everyone else.  All part of the endless-tweaking phase of patch programming.
 
As a loose generality, I find that non-sample based synth patches (for lack of a better term) can usually be triggered from the entire MIDI Note range.  Whether or not that's usable on the extreme ends depends on how you intend to integrate it in your music.
 
The 'real world' sample based acoustic instruments can also be stretched at the highest & lowest samples.  You'll lose the 'naturalness' in short order - usually within a few steps.  Again, that might be usable; depending on how it's applied.

It can all get confusing; especially when you're trying to respond online with any clarity.  You never quite know how your terms are going to translate over the Internet.  For example, I immediately think of convolution reverb in terms of an impulse.  And it was a lot easier when hardware processed with PWM, or by skewing sawtooth waveforms for sound variations.
 
I would personally prefer 'oscillator-based', but those lines have blurred since the carryover from original hardware synths.  Cakewalk synths present unique challenges.  You can create an oscillator from a long 'sample'.  And an oscillator really is a short 'sample' of a repeating waveform.  Waveform-based might be the clearest term, but then you run into the .wav file format that - you guessed it - many longer samples are contained in.
 
RGC used 'wavetable' for the Dim Pro/Rapture family.  Now you run into the confusion with the PPG wavetable-scanning synths.  I don't know what the answer is here.  It's like the preset/patch debate.  What is it - the 'macro' of the entire synth output?  Some portion / element of the synth?  We rarely use patch cables any more.  So, is an LFO 'preset' a patch, or a preset?
 
I guess that's why you rarely see any short postings from me.  I always err on the side of an over-abundance of information; just to get the point across.  Which - in & of itself - leads to more confusion ...

Thanks much brock for your very helpful explanation. I would still like to see some sort of database available to look up the useful or recommended range of Sonar soft synth patches. That would be a great app for someone at Cakewalk to create and would surely be appreciated by users to shorten the trial and error tweaking phase of music composition.