In your case, 47 shows everything but about 75% of the right-most pixel. The Framemeister wouldn't let me custom-define the masking down to the perfect pixel, so that was the best compromise (believe me I tried). If you want to experiment with how much masking is applied on the horizontal axis, what you'll need to do is go into the Visual_Set options and adjust H_Width. This will reveal more or less of the horizontal overscan depending on which direction you adjust the value to. Just remember that changing the H-Width in Visual_Set means you then have to adjust the Zoom_H_Width to re-correct the aspect ratio. Also adjusting Visual_Set H_Width will throw the H_Position off, so you'll have to correct for that as well.
Edit:
I broke back out my Genesis and checked this, and no, you don't want to mess with the current mask setting. Even going one digit lower on the Visual_Set H_Width throws everything out of wack, and it reveals overscan anyway. The current value of 34 is as good as it gets. However, on my set, it's perfectly balanced with the Zoom_H_Position at default of 50. I'd suggest you try a value of 48 to see if it's any more balanced than 47. If it isn't, then I'd recommend sticking with 47.
Also for the record, this appears to be the 5x profiles you're using.
Edit 2:
Also make sure for 256 mode games, you use the 256 profiles. I assume that you are, but I wasn't sure about the Radical Rex comment.
Edit 3:
I took a picture of the game live on my LCD screen to show how the default 50 Zoom_H_Position looks on the 5x 320 profile:
You can see both edges just barely cut into the active pixels. That as close as it will get I'm afraid.