Well, you could raise the over voltage protection circuit to allow the use of higher voltages on the input, the transformer could be rewound to produce higher rail voltages, rectifier diodes could be swapped to FRED style diodes, cap's could be upgraded, op-amp's can be upgraded, potentiometers can be upgraded, the switching devices in the power supply could be upgraded to higher voltage/current devices in some cases, output's can be matched, gate resistors can be 1% tolerance WW dales. The potential list goes on, but the issue I see, is your getting into potentially hundreds of dollars in "upgrades" and the increased output might be just a few % over rated.
The only amp I ever cared enough about to upgrade, has never seen it's potential, it's a hifonics titan TXi750, rated @ 750w @ 1 ohm into 14.2v
Stock we have clamped it producing 800+rms into a 1ohm load cell and 1khz sine wave, testing on a hcca 12.2 wired down to 1ohm, was lackluster @ 760wrms give or take 2%.
I rebuilt the power supply with rewound transformers, re-optimized switching frequency (about 70khz) hand wound chokes, all the rail tracing's and output feed's were reinforced with 14awg solid copper wire, and silver bearing solder, driver circuit got heat sink's on all it's driver fets, rectifier's upgraded to FRED's, irfp3205's in the power supply, rail voltage raised to 125vct, output's matched and re biased to about 7mv DC, removed the stock fuse holders, and soldered in copper bar stock (this can help a ton with fuse ESR when dealing with a amp that has 3+ fuse holders).
The net of this project gained me about 400 more watt's, so iv taken 750 and made 1150, in the process spending about $200 in new parts, for $200 more I could have bought 2000+wrms.
Moral of the story is, amp mods are not cost effective. and on amp's with 4 or more power transformer's, your custom wound tranfo's are likely to be $150 per transformer.