So we know 250 qui gives you 1.5 swing speed (depending on the weapon)

Right now I have 240qui when buffed, and believe I am at max swing speed.
Should I avoid the combat speed buff?

Although I cannot find where I read it, someone said that any qui (or qui + combat speed) after 250 will lower your damage (faster speed = less front load damage) Does combat speed buff further lower your damage? (without the benefit of swing speed because we are at 1.5)

TL/DR Does 250qui+20% combat speed buff lower damage more than just 250qui?

Believing is not good, you should check your swing speed with a swing speed calc.

Haste will lower your style damage. So, if you are really already attacking at max swing speed, you will only lose more damage.

Just take a slower main hand the you shouldn't cap anyway. If you still cap use a slower offhand as well.

3.9r-hand, 2.3L-hand. 250qui, 20% haste = 1.537 swing speed, so, no lost damage.
3.3, 2.3, 250, 10% haste = 1.562 swing speed, no damage lost.
3.3, 2.3, 250, 12%= 1.527
3.3, 2.3, 250, 14% = 1.5 (14% doesnt exist, but is what you need)
So, if you are using the 15% or more, you will lose damage, with no benefit to swing speed.