another important question: do they use a welded sloped sill or a snap in slope or pocket?
you want a welded sloped sill.
Looks like welded sloped by the pic on the website, along with super spacer, foam filling, non-metal sash reinforcement, etc, it looks like it could be solid. Then again, there are other products have those options but fall flat in the structural ratings. That's what I'd like to see... Generally the products that perform well will list the ratings prominently, but that is not necessarily a hard and fast rule.