Max Lead Acid Discharge Current.

I can't answer your specific question with a specific answer, but I can tell you the only way to know the answer would be to experiment under highly controlled circumstances with safety first. But of course, this would ruin the batteries wouldn't it?

Try as I did to find generalized information regarding this, I could not. It would seem that the battery manufacture would know this information if they had their batteries tested by a consumer safety advocate.

I am having a little trouble with your math though.
2000 watts / 12 volts = 167 amps
4500 watts / 10.5 volts = 429 amps
(10.5 being a typical LVD)
Surges of this nature will only last for a few seconds, correct? More than likely your planning includes at least a set of 4/0 cables. 4/0 THHN has a rating of around 200 amps depending on certain factors.
What is the biggest size wire recommended by Xantrex for your ProSine 2.0 inverter, and what is the inverters terminal lugs temperature rating? More importantly, what size fuse or breaker is recommended?

It would seem implausible that one would "dump" a load of 600 amps on just 2 - 6 volt golf cart batteries wired in series for 12 volts.
Something would have to give.
Maybe a bank with more batteries, but common sense would dictate a higher voltage be used if amp loads of this nature where anticipated. Also, this is where proper sizing of and the type of fusing would protect personnel and property.

Even in the world of alternating current electricity, when higher amperages are anticipated, a move to higher voltages is the typical response.
Think about it, why pull the larger more expensive wire for a 800 amp service at 240 volts 3 phase when you can pull wire nearly half that size at 480 volts 3 phase.
I hope something here helps you.

The manufacturer of the battery should be able to tell you how fast you can charge and discharge it.  The more power you push or pull, though, the bigger the wire you're going to need.  I run 400A max through my 12v floor machine batteries in my electric truck, and I use 2/0 cable to connect them.  Golf cart batteries are often used in EV conversions, typically putting them in series for 120v dc @ 400 or so amps.