I have tested with pumpjack with limited amount (modded, so output is uniform)
Example is as follows:
Mining productivity: +200%
1. Pumpjack with beacons and speed module
Mining speed: 1350% (+1250%)
Productivity bonus: 200%
Oil output result: 1620/sec
2. Pumpjack with beacons and productivity module
Mining speed: 1220% (+1120%)
Productivity bonus: 220%
Oil output result: 1756.8/sec

It shows that the bonus of productivity module is multiplied, not added.
Tested with storage tank, and the one with speed module was actually faster.

Thanks for your fancy work!

Are you saying this line (line 157 in version 0.5.0)
local IPS = (product.probability or 1) * (amount * (effects.productivity.bonus + 1) * (playerForce.mining_drill_productivity_bonus + 1)) / sec
should be
local IPS = (product.probability or 1) * (amount * ((effects.productivity.bonus + 1) + (playerForce.mining_drill_productivity_bonus + 1))) / sec
if you can follow that? (I can't guarantee I can follow it)

Adding mining productivity bonus and productivity module bonus, yes. I cannot check the source code right now, but that line seems correct one.
Maybe I can test it before confirming it, but I cannot access my computer until Monday...

I've checked that source code and it shows correct result.

Thank you.

Woops,
local IPS = (product.probability or 1) * (amount * ((effects.productivity.bonus +
playerForce.mining_drill_productivity_bonus + 1))) / sec

this is correct one. +1 is added twice.
I'm currently writing code for this change and other one, and will send pull request on github soon.

Oh I was wondering about the +1's when I took a jaunt through the code; Thanks.