I'm curious about this from another angle, and while it won't matter to the OP with only an extra 10gal, it's relevant to my own summer plans:
I have the C30 mk1 and take a trip most years out on the West Coast. A problem I have had with strapping extra fuel to the rail is that I notice a difference in how the boat rides through the water. Maybe I'm just imagining it...but I don't think so.
Our boats aren't all that heavy by design, and when I start adding weight to the rails (even amidships) it seems to affect it. I have started moving my 2 Jerry cans (40gal total) to the salon floor under the table. I wanted to get the weight as close to center and as low down as possible.
I just think that when the boat was originally designed, they didn't really plan for all this stuff that we now carry on them. Bigger diesels, 2 water tanks, a holding tank, more and heavier batteries. Some people throw a dinghy on the foredeck....most of us carry more chain and heavier anchors than they would have in the 80's, maybe we have an OB strapped to the stern rail, etc etc. The list goes on. I do everything I can to keep the weight low and centre. For long days where there will be a heavy swell running I even move the dinghy OB down under the table...