10 mm sounds like a lot of difference to me.

What made you buy it in the first place? Can you ride the bike with the stock one? If your answer is "No." I'd bolt up the new longer replacement and see how it works for myself. I'm thinking a longer derailleur hanger might allow a stock derailleur to handle a 36t rear cog. Even if it doesn't work it won't be a total loss because you will have learned something new about bike mechanics.