Ok so I have been using the cartridge a bit this last day, even though I am not really fully satisfied of having paid the VAT twice.
Using an original MegaDrive 1 PAL, with a switch to change region/frequency. Always using the correct frequency (60Hz) with all my games. Using 2.00 bios for US/JP.
Night Trap (bin+cue): after 10 minutes of playing fine, I get CD Read Errors continuously and can't play anymore.
Popful Mail (iso+cue+wav): absolutely every spoken dialogue has sync issues, dialogue is cut/repeated/overlapped. It always starts perfectly good, then gets desync after one or two sentences.
Robo Aleste (img+cue): not too sure about it, but I get the feeling that some sound effects are missing. Also, the music did not play at the end of the level.
Did not try other games yet, but this is disappointing. I already tried creating a file .cfg for Popful Mail in which I wrote exactly this :
Did not solve the problem. Activating/Disabling seek time did not solve it either.