Sustained climb rate is not dependent on speed.
Sustained climb rate is also a quantity, a measurement. It can't be argued. From the Bf 109F-4 onward, all Bf 109s outclimb the P51D in a sustained climb at all altitudes.
Ok...the F will because of its power-weight, the G models are some bit of an argument, I'm not going to go there, the K4 I know out climbs pony. Have you heard of Bud Anderson's most rememorable fight he encountered with a 109? If not, here:
http://www.youtube.com/watch?v=FXOCKEotmoU , ignore what they say about slow turn because I already know that's wrong, 109 out corners a pony slow, wait till about 4:20 then it starts. I'm guessing the 109 had to have been a G6, not to sure. Just because all 109s were great at climbing doesn't mean they always won, they say the 109 had better climb at "some altitudes", not all. For example, your saying the 109 can out climb a pony, that's like saying a 109 can out climb a P-38 every time, don't think that's going to happen. You also have to remember that the 109 became more nose heavy throughout the war, so as it's in a climb fight, gets really slow, the plane tends to want to pitch down, hence "nose-heavy", it didn't keep climbing and climbing till the pilot felt like it was time to flip it over.