> Will performance with this option be any worse than using a DISTRIBUTE
> MAIL-MERGE job?

In the general case, yes, it will be much worse. But if you have LSMTP and add USE_LSMTP_MAIL_MERGE=1 to SITE.CFG, it should be about the same. FULL822 is a legacy option that we still support because of cases just like the one you described.  USE_LSMTP_MAIL_MERGE was provided to make performance acceptable when it is necessary to use FULL822. Both are supported options, but their generalized usage is not encouraged.

>Also, are there any plans to enhance the web Mail Merge interface to
>support multipart/alternative messages?

This has been addressed through a new product that will be announced soon. It may or may not be overkill for what you are doing (it is much more advanced than just the feature you are asking about). I cannot say more in public until the announcement.

  Eric