First of all you're dreaming if you think you are going to get 336ft/sec out of any bow at 60# with a 300 grain arrow. You would need less than 35 grains of virtual mass with a 1.4 SEP and that would be assuming a 32" draw and a 7" brace. That would wind up over 90% efficient with a 300 grain arrow. Ain't a bow made will do that. But never mind. I'll use your numbers and plug them in to the model anyway.

Let's see now, according to the model if I take a bow given the following:
vel =336 ft/sec
arrow wight 300 grains
peak draw weight 60 pounds
draw length 32"
brace 7"
virtual mass 125 grains (that's more realistic)

The bow would have to have an SEP of 1.78 to yield that velocity which isn't realistic but like I said, I'll use your numbers.

So if I only change the draw weight to 100 pounds and the arrow weight to 336 grains the model tells me that the new velocity would be 416 ft/sec.