I guess it is not really needed if the program is doing other stuff anyway that takes long enough in cycles before a call to the OPLL-register set function is again being made.
Indeed.
is there already a VGM capture of Great Strategy 2?
Here's my further testing of any delays needed for the YM3812 in my FM-YAM cartridge for the C64.
http://c64.xentax.com/index.php/15-testing-ym3812-register-w...
I can correct it if you have any further data. :)
Still working on reverse engineering the Mr. Ghost and Xak player. Learning a lot.
I think you're quite right regarding the delays needed. For the MSX this holds for the regular 3.5Mhz clocked Z80, however with some MSX's having a turbo mode and especically for the Turbo R different delays are needed. In the OPL4 they introduced a status bit making synchronization on faster systems easy (be it that this chip is already quite a bit faster than the OPL2).