I use a counter pressure bottle filler for bottling from my keg. I would not recommend entering a beer bottled by simply dispensing from the tap for a comp. The counter pressure filler allows you to pressurize the bottle with CO2 and remove oxygen (enemy of beer at this stage). Be sure to chill...
I boil 10 gallon batches in a 13 gallon modified keg all the time. By 10 gallon batches I mean 12 gallons of collected wort and boiling for 60-90mins (depending on what I'm brewing) I come close to a boil over with this much wort at the hot break - but it's never boiled over. I typically adjust...