Last time I checked such things, the bonuses were purely additive (meaning experience_multiplier = 1 + bonus1 + bonus2 + ... +bonusN - penalty1 - penalty2 - ... - penaltyM, minimum of 0, and I think there's a maximum somewhere around 2 or 3). So if you do have e.g. Potential (+0.15) and Scythe of the Void (-1.0), you'll get 15% of the experience a champion without any bonuses would have received. However, it's been quite some time since I checked this, so it may have changed.
