Almost all stat upgrades are multiplicative, adding percentage boni - but some are not. Using the right choice or combination of additive and multiplicative boni is key to getting the most out of your upgrades.
For example: When upgrading attack power, ammunition or AoE, you always need to find the right trade-off between boosting your weaknesses and reinforcing your strengths, since these upgrades will provide lower effective advantages to weaker weapons. But adding range to a weapon is a straightforward affair, and short-range weapons profit far more greatly from it. Using welding lasers or shotguns on enemies ten fields away? Fukc yeah!
Stealth actions are a similar affair. Additional stealth actions can only be gained through propulsion upgrades, whereas propulsion multipliers can be found for almost any part time. Therefore it is important to put as many stealth actions as possible into propulsion slots while percentage-based propulsion upgrades can be found just as well for weapons, shields, reactors and computer parts.
