The new VDP I/O timing emulation broke ExperTurbo support, because the /WAIT feature of the V9958 isn't emulated yet.
Solution: Emulate the /WAIT feature of the V9958, which is enabled by the bit2 of the VDP R#25
Tested on openMSX-0.9.1-506-g6cc03ba, on Windows-7.
Steps to reproduce the problem:
1) on MSX-DOS, enable the turbo using TURBO.COM from this link: http://www.amusementfactory.com.br/msx/getfile.php?fn=turbo11.zip
2) Load any MSX2 game. The VRAM will be corrupted. I tested using King Valley2 MSX2 on scc+ (loaded with SCROM.COM)