I still think that this mod is not a terribly cost-effective path to persue. If your amp is rated for instance at 400 watts continuous, increasing the current capacity of the power supply will do nothing for this static power output. It will effect the ability of the amplifier to deliver it's normal transient power for a longer period, but in my opinion, this relatively slight benefit is not as cost effective as simply buying a higher powered amplifier.

Of course the person doing the mod wants to make some money. I don't know how much the labor and parts cost of this mod is, but I really think you would be better served by buying a new amp that comes with a factory warranty in case anything goes wrong. I seriously doubt that your amplifier would be reparable by the original manufacturer in modified form, nor would it's resale value be as high.