So I just bought a 50% XP boost with the extra station cash I got. Once applied, if I capture a bio lab (1000xp) would that be worth 2500xp (boosts additive) or 3000xp (boosts multiplicative)?
double xp weekend has no effect on base captures. only boosters and membership bonuses are applied. with my 50% alpha boost and (currently) 40% membership boost a base capture is 1900xp (2000xp if membership was maxed out, obviously).
Normally true... but it seems they've changed that now. Anyway, to answer OP's question, you would get 300% xp with the 50% boost equipped. If you added in 6 months worth of subscription bonus (another 50%) you would be getting 400%. The double xp server bonus is essentially applied after any other bonuses you have active, and doubles that total.