A bit complex. . .
 

I have a significant number of Amex MR miles that I want to get into Star Alliance so I can use them on South African. What method gives me the best "conversion rate?"

Thanks in advance.

I recommend using US Airways. You can transfer mile for mile into US Airways from AMEX, and since US Air is *A you can use those miles to book on South African. One particularly nice thing about this option is that you can call US Air to check availability and once they confirm that a seat is available you can do the transfer online while you are still on the phone with them and the AMEX miles transfer immediately, while you wait! So, you never have to get stuck in the trap of transferring the miles only to discover that the seats you wanted are gone by the time the transfer is complete. I should note, by the way, that I have not done this transfer in a while--but it is likely that the instant transfer still works.

To other FT readers--has anyone done this ransfer lately?

To verify whether this is still the case, you can check out our gracious host's Mileage Converter Tool.

No, you cannot (see https://www.flyertalk.com/forum/showthread.php?t=477998 and numerous, numerous other threads).

For the most part, AmEx MR transfers to traditional airlines are 1:1 and MR Options transfers 2:1. The two *A airlines still participating for mileage transfers are ANA and Air Canada.

Thanks for the clarification. I did not realize that US Airways is out of the mix. BTW - It looks like on ANA you have to transfer the miles BEFORE they will make a reservation, but the instructions related to AC seem to leave it an open question. So, it would be worth trying to get AC to make a reservation and hold it long enough for the miles to transfer.

From the AMEX website regarding 1:1 transfer of points/miles to AC:

If you are currently not enrolled in the Aeroplan program, go to aeroplan.com or call 1-800-361-5373 to enroll. Once enrollment is complete, you may transfer points. Please allow 3 to 5 business days to complete enrollment.
Call Aeroplan at 1-800-361-5373 to check seat availability and the number of frequent flyer miles needed for your desired flight. Tell the representative you intend to use your Aeroplan Miles, and ask for information regarding ticketing time limits and policies associated with redeeming Aeroplan Miles.
If seats are available, click the Add to Rewards Cart button above to transfer Membership Rewards points to your frequent flyer account. You cannot reverse a points transfer. You may transfer a maximum of 999,000 points to a single Frequent Flyer account on any given day.

AMEX conversion options are depending on the country that issued your AMEX card. YMMV, so to say ;)

Most European Amex cards can convert to TAP Air miles 1:1.
Amex in UK, France, Belgium, Netherlands can convert to BMI Diamond Club miles, but only in UK 1:1, in the other countries 2:1.