Mostly implemented in commit ddcd3a47a22375a1266a405bf23c6c7b8c0de457 by Wouter (thanks!). But it is not exactly known/sure yet what the value for the delta is on the TMS, see VDP.cc:

// TODO how many extra cycles on TMS99x8?
//  To be on the save side, I picked 10 cycles, translated to
//  V99x8 that's x4 = 40cycles.
// See doc/internal/vdp-vram-timing/vdp-timing-2.html for details.

If I understand correctly, Wouter wants to keep this ticket open until the definitive value is determined (how?).