How much more can you lift in the decline bench compare to your flat bench?
Bullshit. It's 6%.
OP, The answer won't be universal. "A bit more but not twice as much" is probably the correct answer.
Not sure what your aim of the post is, but flat bench tends to come in a many different sizes and shapes. Some people may arch so hard that their bench is practically a decline -- a point to take into account.
And until I enter a meet which includes the decline bench as a lift, I see no benefit in a more accurate answer.
I can probably do more on flat than decline. Mostly because I don't see any place for decline in my training and it kills my elbow.
My decline is almost identical (at most 10-15 lbs. Higher) to my competition style bench. If I don't have a lift off, I'll often sub in decline as the self lift off at that angle puts less stress on my shoulders. However, my guess at my decline max is based on rep work, there's not much point for me to maxing on a decline.