I went from a 2011 RGU to a 2013 CVO RG and that was one of the things I worried about. But, the amount of space lost to the amp and speakers is surprisingly little. Unless you have jammed your saddle bags full to the point of compressing the the contents when you shut the lid, you probably won't notice the difference.
I have a 12 CVO RG Skunk and the 13 CVO Anniversary RK. The amount of space lost is a lot to me. I use to cary on my left bag on the skunk my rain gear, rain gloves, winter gloves summer gloves, various Eye wear, the gps case and my angled saddle bag cooler. Leaving the right bag for my luggage. Well what is in my left bag on the skunk will not fit in the left bag in the King. So it has to go in the right, and I can not totally fill the cooler due to the speaker. I also have less room now for luggage. I can see if you do not take trips on the bike where it would not matter, but when you go for several days or weeks it is significant.