Hi tsialex!
Q: Should I be proactive and get myself a MATT card before my SPI flash fails?
/Best regards Doc
If you can replace it yourself, it's not so difficult, do it next time you take your Mac Pro down for maintenance.
If you can't, then the MATT card is the solution. For people not in Europe that wants the MacPro5,1 going on working, my advice is to buy a MATT card and have it ready for the inevitable SPI flash failure - even the newest mid-2012 is already 10+ years old by now.
Since you are nearby from cmizapper in Europe, the downtime is not something to be as worried as someone in the Americas/Asia/Australia where a MATT card take a month or even more to arrive.