The problem with amps that use the ICs with on-board over-current protection is that using FETs with significantly different RDSon values (compared to the original FETs) is that the over-current protection for the amp will not work as it should. If the RDSon is higher, the protection will trip earlier. Lower RDSon will prevent the over-current protection from engaging when it should. This isn't always a problem but it's something that has to be taken into consideration.