Unfortunately I have been in the same prediciment as you twice before. As much as I always asked myself will I kick myself later I cant regret my decision as a professional and as a business owner. If for any reason you felt that you would not deliver 110% under such a short notice then you made a correct decision. At the end of the day if you have to rush and you are not happy with how it came out then it will show. Clients also do not sometimes understand the amount of prep and hours that go into planning a cake, it does not matter if its 20 people or in your case 300 people.
Kristyne & Sofia