Ok so update on the undervolt issue. The molex connectors arrived so I connected them to a different ATXraspi and voltage converter, plugged that into the system taking out the installed converter and ATXraspi and I get exactly the same result, system stable no undervolt until I turn the volume up around 70% and then I get the same undervolt which slows the games down.
I decided to test some other power supplies, I plugged an official raspberry pi 3 power supply into the ATXraspi and I get exactly the same undercoat behaviour, if I take the ATXraspi out of the system and plug an the official power supply directly into the pi then I get no undervolt behaviour at all even with volume at max. So it seems perhaps I'm losing a bit of power with the ATX in the system, however I tried plugging the micro USB from the converter directly into the pi and I get the same undervolt issue so it seems that the converter itself isn't providing the pi with quite enough power to drive the speakers. I did notice when I measured the voltage of the screen dc cable and the voltage converter that the screen cable is taking 12.1v rather than 12v and the atxraspi is getting 4.9v. I measured the voltage hitting the ATX whilst under load and as the volume is turned up it never drops below 4.9v at the ATX even when undervolting on high volume. Measured at the 5v and ground gpio pins the pi only gets 4.8v from the ATX, it never drops below this even when undercoating at high volume. When I measure the pi gpio pins when using the official pi power, it registers between 5.1/5v and never drops below this which indicates when using a converter and the ATX I get a loss of 0.2v when compared to using the official adapter at the pi gpio I can only assume it's this loss which is undervolting the system at higher volumes. Do you see the same voltage numbers at ATX and the pi gpios on your system?
Based on the above can you think of a way to improve the power provided? Could it be the usb cable to the Picade board? I'm using the stock one but wondering if I need to use a higher rated cable to provide the power needed. Is there a way to measure voltage at the Picade board?
Any thoughts would be greatly appreciated. I know I could just avoid turning up the volume but I'd like to get it working properly and understand where the lack of power is coming from.