just got fat ps3 with matrixps3

I just got hold of hand me down ps3 fat 60/80gb I think it is but it is on FW v3.50 so I bought a matrixps3 USB JB dongle which will allow me to downgrade the PS3.

I understand I need to downgrade it to run JB. But I dont want to run into consequence of killing the BR drive. I ask my friend whats last BR DVD he played in it last he says he dont think he has ever played a BR DVD in it. Obvious choice is to play a BR which has highest MK something file presuming Inception but my nearest blockbusters is 40 minutes away.

So how do I know if my fat PS3 is the non affected 16mb or affected 256mb NAND flash? Is there a way to check before I do any downgrading. My friend bought the PS3 at launch but it was replaced under the one year warranty so I cant tell revision it is unless i open it up to check the mainboard details.