Sickle haybine cutting speed

I'm getting ready to make the deal on a NH 499 I have been looking at but the only thing that is keeping me from doing so is the cutting speed. With the NH 1465 haybine I sold I simply could not cut over 3.5-4 mph and get a clean cut in any crop conditions.....thin or thick. I have read on here where a couple of you guys that are running or have run a NH 499 are able to cut 5-6 mph.....if I knew that it would cut clean at this speed in most conditions I would buy it. I can't help but to worry that it won't be able to cut any faster than the 1465 though. What is the key to getting a sickle haybine to cut at higher speeds? Do the type of guards have any influence on cutting speeds?

Sickle haybine cutting speed