The bench looks very interesting and seems like it can be used in a variety of ways. Here is what I recommend for relief carving - height should be about elbow height, or just below elbow height. Lower than this, you will need to bend over too much, and higher than this, you will strain your shoulders trying to lift yourself higher. The design looks like it has a lot of holes for bench dogs/hold-downs, which is great to have the flexibility of many ways to hold a piece. I have a carving bench that I purchased from Woodcraft about 10 years ago that has a top that tilts (as in the design). However, I have never used the table tilted. I find that working flat, I put a lot of my weight into the carving, and if it were tilted, completely different muscles are used. I would guess you could continue to put your weight into the carving up to about a 45 degree angle, but when it goes more vertical, it's a whole different process. I have had to do carvings on a vertical surface and because my muscles have not been trained for this, I was exhausted after about an hour.
Just some thoughts. Happy carving!