I've had lots of problems with mirroring in 4.X sites. As you say, it works fine untill you start pushing large amounts of data... Then things start failing and the servers run out of memory etc. (You should enable logging to find out exactly what your problems are)
I haven't tried mirroring in R2, but I know it has been totaly rewritten for the R3 release. So, one solution could be to start migrating the site to R2 code, but wait for the R3 release to go live?...
A customer has set up mirroring between two EPiServer 4.62 sites. The customer has experienced many problems with the mirroring, probably because of large amounts of data (many and relatively large files linked from the mirrored pages). If an update contains more than a few hundred MB the mirroring crashes. I don't know if this is because of the amount of data itself or because of the time it takes to handle. Has anyone else experenced this problem?
The plan is now to migrate the sites to CMS 5 R2. Has anyone experienced the same kind of problems in this version, or has mirroring improved (I know it's completely different and has to be set up from scratch after migrating)?