What are the cons to an event like this. ?
Well, one con may be that according to the JPO pres, you can't simply decide that you're going to fly outside the AMA rules one day, on the basis that you're not at an AMA chartered club (etc), and that you have no intention of making use of the AMA's insurance that day. The theory is that even if you have no intention of using the AMA's insurance while you're doing your speed event, supposedly there's nothing stopping a plaintiff in any accident going after the AMA anyway (if you are in fact an AMA member), so that if something happens and the AMA gets sued then the pilot, event organizer etc could presumably all lose their waivers.
Some other folks have indicated though, that they've been able to contact the AMA and effectively suspend their AMA membership for a day or two, and that the AMA folks were ok with that.
Who knows where the real truth lies in any of this, given the various private agendas that seem to crop up periodically ... but you could perhaps try sounding the AMA out about temporarily suspending memberships for attendees.