I know a lot of people say that marksmen feel weak because of itemization, but I just don't see it. Any way you shake it, ADCs are hitting their appropriate powerspikes, and doing damage accordingly when allowed to.
There's the caveat though. "When allowed to". Marksmen are intended to be a frail DPS class that can survive as long as their tank is able to stand in front of them and soak the damage that would kill them. Here's the problem. The front line is completely irrelevant. Vi completely ignores it, Nocturne completely ignores it, and in most cases, an ADC has to get too close to threats to do their damage.
This is a huge balance issue. Before looking at buffing Marksmen and Lethality, I would seriously implore you to consider that ADC weakness is a product of environment, not lack of damage.