We've already created a backstory for an even more heavily up-gunned LAV-75 (see link). Perhaps you could work off what we've got. The M20 Ridgway write-up included a bit about the 75mm gun being underpowered against the latest generation of Russian MBTs. Perhaps the 90mm gun was an interim attempt to increase the effectiveness of the LAV-75 main armament vs. Soviet MBTs without adding too much weight.
http://forum.juhlin.com/showthread.php?t=1043&page=12
A PDF of my M20 Ridgway write-up is attached to one of my messages on the last page of the thread. I hope you can make use of it.
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