I play a 255 server. Would a +10 lvl1 Weapon have a big difference from a +7 lvl1 Weapon in regard to damage?
That depends entirely on your weapon level.
Weapon lvl1 +2 atk per upgrade
Weapon lvl2 +3 atk per upgrade
Weapon lvl3 +5 atk per upgrade
Weapon lvl4 +7 atk per upgrade
(Also increases Matk by same amount)
Once past the safety limit the bonus will differ (like +0-14 Atk per upgrade on lvl4 weapon).
Every upgrade of +11 to +20 gives also different bonus that I don't know never had such a weapon.