In Sky Force 2014, is there any way to see the details of hangar upgrades? E.g. I like to see what improvements would upgrading the main cannon brings (like "+10% strength, +20% frequency").
No, actually there isn't a way to see these stats or even if the upgrades consists of percentages, they might consist of points, we just don't know.
I can tell you what the maximum level each upgrade goes to.
Health: 5 Levels (So the 50 upgrades)
Main Cannon: 4 Levels (40 upgrades)
Wing Cannons: 5 Levels (50 upgrades)
Magnet: 4 Levels (40 upgrades)
Missiles: 5 Levels (50 upgrades)
I'm not a 100%, but you have to remember this is always changing as they keep expanding the game.
Maximum level upgrade in v1.33: