The theory is that each hull performs best for a specific design displacement (e.g., how heavy the paddler plus any load is). So, theoretically, a person in a boat designed for their weight will paddle faster than in a boat designed for someone else. They may be also more stable compared to a lighter or heavier person in the same boat. Fit and comfort are another factor, which is difficult to achieve by just padding. And ergonomics have to scale with the paddler too - you can't make the catch area of a big wide boat designed for a heavy person as narrow and low as you can for a smaller boat...
Will you be faster on flat water in a L compared to a regular V10? Probably. The V10 is made for bigger folks (not huge and overweight, but above average, I'd say). Too much boat for a smaller/weaker person in many ways. The V10 seems to require a good strong engine on the flats - the V10L should be a bit easier to move through the water for a smaller/weaker person, I would think.