Pretty sure that blocking/bracing would be required to meet code here on a 15 foot span.
At least 1 set, possibly 2 lines of blocking.
Well my joists are Douglas Fir #2 which according to the span tables can achieve the code required 10psf dead load and 40psf live load ratings at a span of 15' 7" so it is well within code. I just know that this rating works well if the lumber has no holes or notches in it and once those exist the rating will no longer be quite on point.
You could measure the deflection [sag] in fractions of an inch at midspan before and after putting a concentrated known load [two or three people] at midspan.
From this the E value of your existing joists can be estimated and the whole thing reverse engineered, within some error.
Blocking prevents the joists from twisting and so you get the calc. value of stiffness. If a joist twists the formulas do not apply.
Performed this experiment with a total of 470 pounds and saw less than 1/16th of an inch difference. Clearly the floors are strong.
However, for the total cost of $330 (150 feet of 2 x 10s plus labor) I've decided to go ahead and do this. The framer starts work on Tuesday. Thanks all for the input.