Thanks all for comments and advice. If EFCT is also known as CAE in DXB, then that is the first compliment I have ever heard about that place!
I completed G550 Initial at FSI LGB in 2009, did the 450/550 diffs at the same time and have flown both since. All *seems* straightforward for keeping the FAA happy in this regard... but can anyone point me to a reference that states clearly that an Initial plus differences is all that is required to fly both?
Cheers