Valuation Question-Comparable Multiples

Hi guys, I had a question on comparable multiples.

You are trying to value a stock or company and are using comparable company multiples. The company (Company X) is special in its sector where no other company is really similar to it. (ex google). You come up with a list of lets say 5 companies in the industry that are most similar to Company X in terms of the desired characteristics (rev growth, product, market, eps growth etc). You use a fwd P/E multiple and come up with a median/range (lets say the range comes out to around 16-18x, with a median of 17x). Company X is trading at 40x fwd P/E.

How would you justify if Company X is a buy or sell based on this valuation method? Company X is trading at fwd 40x P/E but the industry average is 17x. The bears think Company X is overvalued because it is trading so far above the average. The bulls think that Company X still has upside. For each side, what exactly would you do to assign the proper multiple to Company X?

For example lets say that on average your comps revenue growth is expected to be 10-15% on a 5 year basis. And for Company X, revenue is projected to grow on at 75% on a 5 year basis. Is there any method to really assign a proper multiple to Company X? Obviously Company X is growing at a much faster rate than the average. But how much of a higher multiple does it deserve?

Thanks for your help. Valuation is not my cup of tea.