The FAT in the PSP MC is very similar to that of the PSP MQ however it is designed in the way that it doesn't change the original signal at all if there is not filtering (MQ) or compression/expansion (MC) engaged. However if the filters are engaged or compression/expansion ratio is different then 1:1 the processing is done with minimized digital artifacts. You can clearly observe this by doing some measurements.
You can also measure the linear response of both plug-ins to note that the brightness mentioned by you is probably a psychoacoustical phenomenon.
This doesn't of course mean that the sound change with the FAT engaged will sound better in a particullar situation, it is just an option to choose from. For me the PSP MC sounds good (and measure as well) great even without FAT engaged and in many situations I would choose to process without it - especially in high sample rates where there is probably no further need to double sample.
Regards,
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Mateusz Wozniak
PSPaudioware.com