Isn't it a good idea to use clamdscan in ISPConfig instead of clamscan? As you mentioned, it is much faster indeed. And nobody would have the quota problem anymore.
Yes, thats my idea too. The main problem is to find a way to not break any old installations and every linux distribution has its spamd in another location so we will have to deliver and run a own spamd as we currently deliver the clamscan binary.
The behaviour that clamscan unpacks the files in temp instead of using a central database must have been introduced with version 0.90 as we never had these problems with the 0.88 versions.