It'll run fine, but I think OS 9.x might throw a fit. Depending on what the firmware extension does for the Sonnet card (i.e., give details about card info, serial number, crucial software for compatibility, etc.), I can only guess what could happen. For example, I've run a Sonnet Crescendo G3/500Mhz card on my Power Mac 7500 for years with no Sonnet software and had no issues. But bear in mind this is a totally different model than what you are dealing with.

I still think you have some crucial software extensions, firmware upgrades, etc., that are missing for this to work properly.

73s de Phreakout.