I dont have a merc engine at all - but i stumbled across this web page of a fishing boat review in googles cache (the page has since been removed from the web) It talks of the merc 115 2 stroke - and the review is from 2005 so it must be the carb version also.
Anyway - on the fishing boat the review states that at 25mph they were burning 4 gallons an hour and at 35mph (flat out) they were burning 6 gallons an hour
http://72.14.203.104/search?q=cache:...&ct=clnk&cd=39
EDIT - thinking about it though - that doesnt sound enough for a 115 carb 2-stroke surely it should be nearly double that.