@Dave S Changing that value made no difference.... except to make the main menu and dock change colors
You should notice overall less transparency throughout the system. These values do affect a lot of subtle things that are sometimes awe-inspiring how thought out they are. For example, somewhere along the way, some moron introduce jiggly toolbar items when customizing them. Reduce Motion makes this not happen like the good ol days of 10.11.
You should see subtle changes like sheets are far less transparent and menus should be less transparent. The menubar and dock as you noted are significant and easy to notice. It's a "reduce" transparency setting, not a "disable", so you will still see some things have a little transparency to them still.
Side note, I also recommend upping the contrast a smidge for Mojave - even if you've never used this accessibility feature before, it makes Mojave significantly easier to see.