well to answer your first question. euro bracing significantly helps the pains from bowing. Think of it using paper. You can bend a piece of paper really easy , but as soon as you tape a piece of paper along the edge( or bend it at a 90 degree angle) it becomes almost impossible without kinking the paper. Typically bracing is on the top and bottoms of the tank. Euro bracing is a great method if you want to use a little bit thinner glass on an all glass tank, but comes with a few problems. You can no longer hang anything on the edge of the tank, and your open area is a little less. To me the benefits out weigh the cons. But I have never had a Euro braced tank...
Now to answer your question about the 8mm glass... You are a touch on the thin side for my taste. Not because of the length of 40" or the width of 20" but the height is my reasoning. once you start getting closer to the 24" mark you start to develop a lot of stress on the vertical corner seems about a half to two thirds down the tank. the glass essentially begins to want to bend along the horizontal axis. By using thicker glass, it not only reduces the amount of bowing in both vertical and horizontal axis, it also provides you much greater area for the silicone/rtv to bond the panes together making much stronger corners for taller tanks. I have a 24 inches tall tank built using 3/8", a 65 gallon. But its 36" long and only 18" wide. The extra 4 inches in length, 2 in width and 4 in height add a pretty good amount of water. And these tanks justify using at least 3/8" (10mm) not 8mm. So me personally, I would shoot for either a fully euro braced 3/8" thick tank or Ideally using 1/2" glass and use a single glass center brace rather than a full euro... I dont think these tank dimensions would need a full euro with 1/2" glass. Just my thoughts.
