I was looking for something sort of similar a while ago for some reason, and found Greg Sandell's
SHARC Timbre Dataset:
"SHARC is a dataset of musical timbre information that I collected by analyzing over 1300 orchestral musical instrument notes. Specifically, the information is amplitude and phase data from a selected steady-state portion of each note. The dataset is now available in XML format."
Assuming no XML-phobia, the data seems usable enough.
However, if your goal is to create something natural-sounding, the exact timbre is probably of comparatively less importance than the way timbre and volume change over time (and that data is not present in SHARC, I think). In practice, I don't think additive synthesis necessarily has an inherent advantage over subtractive or FM synthesis in that respect--your own ability to program it, and especially with a view to envelopes, is more important.
For bottom-up patch building, that is, as opposed to resynthesis of an existing recording. For the latter application, an additive synthesizer with a separate volume envelope for each oscillator can, understandably, be very successful in reproducing pretty much any sound--IIRC the Synclavier, for instance, used additive synthesis rather than sampling. If you want to do anything with the sound, however, I would think that a massive amount of editing is necessary, compared to other methods of synthesis.