So after slicing hundreds and hundreds of mods I was curious about what stats I was getting upgrades on (which secondary stats were getting the most upgrade rolls). So I wrote a quick and dirty page scraper to pull all my mods from swgoh and breakdown the stats for me. This is the result:
Mods for user 831536898, Total Mods 1091
Stat: Defense, Percent of rolls: 23.21063%, Possible: 978, Actual: 227
Stat: Speed, Percent of rolls: 31.65563%, Possible: 3020, Actual: 956
Stat: Health, Percent of rolls: 21.81656%, Possible: 1123, Actual: 245
Stat: %Protection, Percent of rolls: 24.8394%, Possible: 934, Actual: 232
Stat: Protection, Percent of rolls: 24.61539%, Possible: 1040, Actual: 256
Stat: %Health, Percent of rolls: 24.79508%, Possible: 976, Actual: 242
Stat: Offense, Percent of rolls: 23.34241%, Possible: 1101, Actual: 257
Stat: %Defense, Percent of rolls: 21.35076%, Possible: 918, Actual: 196
Stat: %Potency, Percent of rolls: 23.47418%, Possible: 1065, Actual: 250
Stat: %Critical Chance, Percent of rolls: 22.53642%, Possible: 1167, Actual: 263
Stat: %Tenacity, Percent of rolls: 23.61377%, Possible: 1046, Actual: 247
Stat: %Offense, Percent of rolls: 21.56398%, Possible: 844, Actual: 182
The "possible" values are the number of times something could have rolled on the upgrade. The "actual" is the number of times the given stat got rolled. And the percent is the actual / possible. This means a gray mod would result in 0 possible, and 0 actual, cause gray mods have no upgrades yet. A green mod would have produced 1 possible (across four different stats) and 1 actual on one of those stats. etc up to a gold mod with 4 possible (across all four different stats) and 4 actual stat increases.
The reason my speed upgrades rank so high compared to others is likely due to my having sold numerous mods that started at 3/4 speed, and got no upgrades when leveling to 15 - e.g, I sold a lot of the worst mods in terms of speed.
If anyone is interested, I'll run the script for the first handful of ally codes people send me, if they want to see their own breakdown and post the results here. Obviously you need to have a swgoh account, and this will only count equipped mods.