The Macs that had SCSI chips added were the early pre-Plus Macs. In most of those cases the old host Mac had the new Plus ROMs (or 512KE ROMs) such that support for the SCSI chip was already in the firmware (ROM) even though the chip was absent.
In some very early cases an extra ROM (EEPROM) chip was added which contained the firmware needed for the SCSI chip.
In order to change the existing SCSI chip on a Mac, it doesn't just need to be pin compatible, it also needs to operate in exactly the same way, such that the firmware that operates it will still work. The registers also need to be at the same addresses, as CHC pointed out, and these two things can be related.
The SE/30 uses the 53C80 SCSI chip. The next step up is the 53C94 or 53C96 and it's not really substantially faster. After that, one gets into SCSI enhancements such as Fast, Ultra, and/or Wide. At that point, newer firmware would definitely be needed.
A faster chip could be added as a PDS card, but at this time, if one were going to build a PDS card, and was going to have to write firmware anyway, one might as well design a SATA card or maybe a USB card.
In some very early cases an extra ROM (EEPROM) chip was added which contained the firmware needed for the SCSI chip.
In order to change the existing SCSI chip on a Mac, it doesn't just need to be pin compatible, it also needs to operate in exactly the same way, such that the firmware that operates it will still work. The registers also need to be at the same addresses, as CHC pointed out, and these two things can be related.
The SE/30 uses the 53C80 SCSI chip. The next step up is the 53C94 or 53C96 and it's not really substantially faster. After that, one gets into SCSI enhancements such as Fast, Ultra, and/or Wide. At that point, newer firmware would definitely be needed.
A faster chip could be added as a PDS card, but at this time, if one were going to build a PDS card, and was going to have to write firmware anyway, one might as well design a SATA card or maybe a USB card.