I must say that when I tried the Cutek Extreme wood preservative on my pc. of weathered thermally modified wood decking the results were amazing. I applied the Cutek Exteme after washing down the deck boards with soap and water then I let dry. I then applied two coats of the Cutek and I used the Walnut stain option for the colortone. I was expecting something close to the original color when I applied the Cutek but once it dried up the surface was slightly darker then expected but I found out after researching online that applying stain to a pc. of wood that has weathered will not come back to the exact original color because the natural surface pigment has been altered due to the suns UV. I liked the Walnut color choice I had made and because Cutek is a fully transparent oil based wood preservative I was able to still maintain the natural characteristics of the wood which I really liked.
The thermally modified wood that I used was made from a species called North American Ash and it had lots of grain pattern which I found out was a common trait for this species as it has an open grain pattern similar to Oak. The decking stood up very well over the winter and I could not see any pcs of the decking that needed to be replaced. I purchased my decking from a Home Hardware store who was a dealer for the thermally modified wood and they also recommended that I try the Cutek Extreme which they sold.
Overall I was very happy with the outcome. I would recommend that if you do purchase thermally modified wood for your deck project that you apply some kind of oil based wood protection soon after installing or before its installed. It does gray fairly quickly but then again some people apparently like the gray patina appearance.