How large of a DF delta for a run-away season?

[Zynerji]

Would a team need 5% more (with relatively equal power)? 10%? Or is it as low as 1%?

[DiogoBrand]

I don't know about downforce, but let's take weight for example: 795kg if I'm not mistaken is the minimum weight including the driver with an empty tank.
7,95kg (1%) would probably not be enough to give you a free championship, but 39,75kg (5%) would probably do the trick unless you screwed up badly elsewhere.

I think the same can apply for downforce.

[Stu]

Would a team need 5% more (with relatively equal power)? 10%? Or is it as low as 1%?

I would go 1%.
If you work out the lap-time difference as a percentage (for last year) 1% would put you out in Q2 at best. If you go back to 2014, you could be almost 3% off if you had a Mercedes PU. Engines have been far closer in the last two years.

[dans79]

More downforce by itself doesn't mean anything. At what speed you generate the downforce is important, as is the associated drag you get with that extra downforce.

[NL_Fer]

Even with a significant downforce efficiency at the 160-250kph range. On half of the tracks the car with superior rear traction and power can win it.