In looking around the race paddock, I'd say that most of the HTA guys still prefer a period Koni double adjustable shock. People like http://performanceshock.com/
(PSI, at Sears Point) can do a wonderful job restoring/rebuilding them to proper race spec as well.
Performance Shock can also build new double adjustables, that look and perform just like the originals as well. They will not however have a period date code (if that matters).
You might also consider looking into whether http://www.truechoicekoniracingservices.com/
can help you with some new Koni shocks. As was discussed previously in this thread, True Choice has been able to build new versions of their old double adjustable shocks and it's possible they are still offering this service.
As for what other shocks competitors might be using, I honestly don't recall. You will not be dissatisfied if you were to go the Koni route.