Just go for it! But, a good probe thermometer is an absolute necessity. I've rotied a 6# boneless leg o' lamb, two 14# turkeys (separately - not at the same time...) and an 18# boneless prime rib recently, and all of them took around 2 hours - and were perfect. It was difficult to get the dome temp below 400F, even when I started with a moderate amount of briquets (you can always add more, if necessary) and closed the bottom vent(s). But don't let the high heat scare you cuz as long as the spit is turning you're heating evenly. Which is why you need a good probe thermometer - you need to monitor core temp accurately so's you don't overcook.