Yes they are. Reinforcement is a great feature as it can prevent the window from warping although both of the aforementioned windows use a very high grade uPVC so warping would be rare either way. However, the true reason is to prevent deflection which is caused when a gust of wind causes the top and bottom sash to separate ( ever so slightly) at the meeting rail where air infiltrates causing a draft. Sash reinforcement along with a high quality extrusion will prevent that. Thats one of the main reasons both have such low air leakage rates. Further, the hardware is mounted into 2 or 3 walls of pvc PLUS the reinforcement which adds strength.
Keep in mind, both Okna and Sunrise use composite reinforcement and do NOT use conductive metal which could not only negatively effect the uFactor but also cause condensation inside the sash.
So the short answer( lol ) is reinforcement is a great feature.

This post was edited by mmarse1 on Sat, Nov 2, 13 at 22:52

When you say composite reinforcement, is it similar to composite decking material or a type of fiberglass composite?

Actually, there are both.

Hi mmarse1: Do you mean Okna and Sunrise use different types of composites in the manufacture of the windows or they both use different types of composites in their windows?

They both use a different material.

Great info from mmarsel above. The reinforcement in both of those products improves performance as evidenced by the change in DP rating.

Thanks mmarsel.