Well “reduced spread by 15%” doesn’t necessarily mean you’ll be getting 15% more hits. It’s impossible to know exactly what they meant by it. But even if they did mean that, you wouldn’t need a %15 reduction in damage to keep it the way it was before. You would need more like a %13 reduction in damage. Think about it. If you get an %100 increase increase in the number of bullets that hit, you don’t need an %100 reduction in damage to match it, you only need a %50 reduction. IF they did mean %15 more bullets will hit your target the relevant value is not the percentage of the increase, but the ratio of the increase over the original value. For example, lets say say 100 bullets hit your target in the old system, and with an %100 increase in accuracy 200 bullets hit your target in the new system. The ratio of the increase is 200/100. For the overall effect to stay the same, bullet damage will have to be multiplied by the inverse of that ratio, 100/200. Likewise if accuracy was increased by %15, now 115 bullets will hit your target, the ratio of the increase is 115/100, and the inverse of that is 100/115 which is about 87%. In other words a %13 reduction in damage is needed for the effect to stay the same.
I hope that made sense. With the lack of more detailed information on the tweaks the best way to compare “gun effectiveness” is really to just play the game.