I have had the new Golden uMax with stock 9X8 for about 3 months and just recently purchased the 5.75 concentric. I have not use the 8 inch round so I can not compare.
What I can tell you is the 5.75 is amazing. I will probably never take it off. First off, it noticeably made the unit lighter. The short time I have had the 5.75 I have noticed this;
1. Depth is very similar to 9X8. At least I have not noticed a difference
2. I love the 5.75 concentric
3. Target separation is fantastic. I truly believe my treasure to trash ratio has improved
4. I love the 5.75 concentric
5. It may seem like you will take longer to cover a specific area (volley ball court, playground, etc) But I think by separating the junk you actually cover the area in about the same amount of time as with the 9X8
6. I love the 5.75 concentric
In the short time this is what I have noticed. I am sure more will surface as I get more time with it.
Sorry Tabman I had to steal a page from your book