Hello Sandy,
I'm the designer and builder of the Think Evo. Most folks who have paddled the Evo feel its stability is very similar to an XT. Some think a bit more, some a bit less. The gunwale is taller, but this is just helping to keep waves out of the foot well, and most likely not affecting stability.
The main difference between the Evo and an XT is in the length and rocker. The Evo is a half meter longer, and has a little less rocker, with sharper entry and exit lines. The result is greater speed, and glide, allowing the Evo to compete with faster boats, like the Fenn6. This is especially true in rough water, as the increased stability offered in the Evo results in greater power and efficiency.
Please let me know if I can answer any other questions for you,
Daryl