|I haven't answered this because my experience of this setup is old and I didn't want to say something I mis-remembered; recently I used this system, which has refreshed my memory and added a couple of new dislikes, so here goes...
Double-tackle systems should work reasonably well on a broad reach to a run, but one of the goals for most owners is to simplify the rig, so there's no vang - a poor omission in my view. The main turns into a shapeless rag. Once you come up to closehauled, the appropriate mix of boom position athwartships and leach tension requires a degree of fiddling that most people can't achieve with ease - or want to bother with, so the boat doesn't go to weather as a C&C27 should. You might as well be sailing a Catalina. You also end up with a lot of line in the cockpit, which defeats one of the objects, which is keeping the cockpit clear.
If you want to clear the cockpit at the dock, put a ring on your traveller and a snapshackle on the lower mainsheet block then shift the block out to the rail when you want more space. The double tackle on the coachroof is a clumsy compromise.
C&C 27 Mk IV