After reading through the Web site, it seems as though the Siremax name is just a 'brand' that the flock owner calls his sheep. It is essentially a composite (crossbreed) with years of performance records. One thing that got me, though, is that on the Web site, they state 'LambPlan EPDs' -- in fact, it's really LambPlan EBVs. EPDs are the evaluation tool created by the NSIP (and, to my knowledge, are used differently).
Anyhoo, this flock reminds me a lot of the Tamarack flock, except the Tamarack flock advertises BOTH maternal and terminal traits whereas the Siremax flock advertises JUST terminal traits.
Personally, I think Siremax rams are an outstanding terminal sire with performance records to back up their carcass characteristics. (It also looks like the Siremax flocks only sell rams.) If I got a ram, I would cross him onto a flock of maternal-bred ewes and market all the lambs. Not once did they mention the maternal characteristics on the Web site, so it would be a shot in the dark if his daughters would be productive ewes.
This is totally terminal. The siremax people market all the ewe lambs except for the replacements. These are are run through the EBV program. After talking to John Essame, the ewes are still good mothers, just really breeding for the ram sale. Its based on performance.