Depending on the year Ultegra could be 9 (6500) 10 (6600/6700) or 11 (6800) speed, what the OP has been is looking at is key, given the 2-3K on the used group, no reason to believe it couldn't be the 11 speed, as that has been around long enough to have that sort of distance on it, even for a lightly used groupset.

For upgrading from Claris, if the bike has Claris, that would indicate that it's a lower end bike, with the associated frame (the CAAD 8 frame actually look pretty nice), wheels and other components, just sticking a higher end groupset won't make anything else any better. Claris is a new product for 2014, and although low end, is one of the nicest looking entry level groupsets Shimano has put out, would keep it as is, and look to upgrade to a better bike once the CAAD 8 is worn out, rather than changing what is a good groupset to one which may have compatibly issues.