I have fixed many scratches with water in the tank. Definitely not as easy as being empty and a longer process but it can be done. As for yellowing I got my 500 and 700 gallon acrylic tank used. The 500 was close to 7 years old when I started(4 years ago) and the 700 was 10 years old(3 years ago) and they both look clear as the day I buffed them out....
After reviewing all glass tank manufacturers, Aqua Vim tanks seem to be on top of the lists. Their web site shows variety of styles and sizes, and their 2,000 gal and 1,000 gallon custom tanks built using impact resistant glass shown in their "Super Custom" page are extremely impressive.
I am going to visit their facility in Maspeth, NY some time in a week or two.