I'm actually very very bothered by "0.3% of the speed of light". This is 900,000 meters/second. A passenger airplane flies at slower than 300 meters/second. Harry is flying 3000 times faster than an airplane?

Let's say a broom accelerates a thousand times faster than a car. A car can go to 60 mph in 7-8 seconds. Let's say 5 seconds. That's an acceleration of about 5 meters/second squared. Let's say Harry accelerates 5000 meters/second squared (an impossibly large acceleration). It would take him 3 minutes to get to that speed. All the while experiencing 500 G force, that is 500 times the force of gravity. The record G force survived by a human is 46.2.

Seriously. This is 9000 football fields in ONE SECOND. He can't possibly be making those turns and bumping the Weasleys like a bludger. He'd be killing them all (in a fraction of a second). Throw in some more zeroes please! 50 meters/second (about 100 mph) is the limit of what I find believable.