For BenQ 1620 and 1625 (LS), it was not just an hardware problem.
Crossflashing is possible between the two drives, and is working for CD & DVD burning. But a BenQ 1620@1625 can't do LS due to some hardware part missing.
BUT maybe one of this LG model (4164/4165 or 4167)
is a 4166 with a restricted firmware, who knows ?
Some external LG GSA-2166 are marked 4164 inside and newest ones are marked 4166...
But they are so close in price that the interest of crossflashing seems very low to me.