hey mate you will be fine, check out the modiction anvil and Blox shift knobs all around 500gr, and been used for years. mine is 487gr stainless.
you can flick the knob into gear with little effort due to inertia and i love it, i think it would be great with a short shift kit that sometimes get a little notchy. yes it adds weight but i believe with something that weight you will never miss a gear so that makes up for the 500gr, one missed gear would cost more time.
it possibly would wear the bush quicker but even on the U.S forums no one can answer that, ill brass bush my shifter turret next time it gets a rebuild but to date no ill effects to speak of.
my gear knob is similar size to factory and i tapped the shaft lower to get it down to standard height, the factory NA8 was about 200gr i think but don't quote me on it.
oh and its a nice piece of kit you have ended up with there
