I think the main problem is that achievements are additive to the bonus you are already getting from the achievements. If they were multiplicative, I would not be upset or mad about the achievements in the slightest. (mind you I am not mad or upset about the achievements, I am just thinking its a shame that it isn't multiplicative at this point, maybe an extra reward for completing t5 11th objective? making them multiplicative?)
We don't currently plan on having any multiplicative bonuses from achievements, but I'll add your vote to the item on the suggestion list.