No, it's not a huge difference, and the numbers are not quite right.
20" 343M are 20x9 ET34 and 20x10 ET34. (19" 345M are 19x9 ET32 and 19x10 ET34)
Non-M F10 19" and 20" wheels are 19x8.5 ET33 and 19x9 ET44.
So the front is no problem, rear is more aggressive but will still fit, at least with OEM size tires.
And the M5 wheels are forged and very expensive.
