The +7, +14 ect do not represent any specific distance. I believe that the maximum that can be added by apparel is +100 so it’s just a number to show where you are on the 0-100 scale.
As far as the actual distance added, for Driver/3Wood/Iron distance increasing apparel it will literally change the distance show when selecting the club. So a driver that normally has a distance of 285 yrds will show (for ex) 302 yrds when the apparel is active. You may need the adjust your distance or even remap you club if you are using distance balls.