Just had a quick look at CPU benchmark website for the scores.
i7-6700HQ (2.6GHz) - 8046
i7-6820HQ (2.7GHz) - 8717
i7-6920HQ (2.9GHz) - 9595
The 2.7Ghz is 7.6% faster than the 2.6Ghz.
The 2.9Ghz is 9% faster than the 2.7Ghz and 16% faster than the 2.6Ghz.
In real world usage will that make a difference? All the basic stuff won't be any quicker, unless you do some massive render, compile or conversion that 16% quicker will actually benefit.
Rookie question, but would a cpu upgrade provide any noticeable performance benefit to gaming?