When we had a stackpack fitted to Delphine, the rigger recommended attachment to the mast rather than the spreaders, on the grounds that the loads could be quite high and the mast was stronger. We accepted his advice - but there is undoubtedly a penalty when raising the main that the main sail battens can easily get caught on the lazy jacks. In daylight, when you can see what to do, it's manageable, but trying to get the main up before dawn last year took ages!
We run the tension line for each lazy jack through a turning block tied in to a cleat on the mast, then along the boom, and back to a jam cleat with 2 to 1 purchase on the aft end of the boom. This makes it very easy to take the tension off the lazy jacks to sail, and to put it on again so that the sail is contained as it's dropped, all this being done from the cockpit. It really
is necessary to be able to adjust the lazy jacks as the two situations of containing the sail with the boom high and the topping lift on and not interfering with the set of the sail when sailing require completely different tensions. We also had a system last year with another line that could be used to pull the lazy jack on one side forward so that the battens didn't get caught, but we haven't reinstated that this year - we may do so.
Delphine, Parker 275, no. 41