Extusion rate limiting and firmware retract
Sadly this does not work at present. Setting an M203 E3 resulted in 6 mm retractions via G10 taking 2 seconds each vs the fraction of a second they're supposed to take.

Since it seems desirable for this to work, I've filed a Marlin issue to see if it's intentional or something that can be fixed.

In principle, G10 firmware retractions still make it easier to script a soluton (replacing each G10 or G11 by a sequence of M203, G10/G11, M203 to set E-axis speed then reset it around the operation) but this is less friendly to direct usage of same sliced files with different extrusion speed limits.