A group size of 512 does not bring significant improvement (less than 1%). The DRS62 ModInv is fast and almost negligible with a group size of 256.
If you have a modular mult faster than the digit serial Montgomery mult on a 256bit field, I'm obviously fully open. A folding does not improve thing on 256 bit when working with 64bit digits. I'm not sure if Barrett could be faster, I must say I didn't try and for "medium size field", there can be traps.