Okay, I have had both kinds, a Nacra 5.5sl and a prototypical A-cat (both without booms), Nacra Infusion (a boomlet!) and here's my 2cents worth:
For recreational cat sailing, the boom is an over-rated component of the boat. What performance you can derive from the boom in everyday fun sailing conditions probably won't ever be noticed. The out-haul on the boom *does* enhance the performance of the boat, however, by providing additional tuning capability, especially for high and low wind conditions, but the effect is marginal. How? In high winds, the boom provides a direct ability to depower a bit more effectively, by use of the out-haul, which can flatten the main-sail and allowing for more tuned power. The boomless rigs wiggle and arc more in gusts, which can mean more of a deeper mainsail foil shape, but its really not a big performance deal. If you were to sail both in gusty air, you would probably find you would be wiggling your helm more in the boomless rig to compensate for power spikes and aacceleration, and your cunningham (down-haul) would be honked down hard on the boomless and only tweaked on the boomed boat. In light air, it similarly related to sail shape. On the boomless, your bottom batten will straighten the mainsail more than a boomless, (which may or may not be preferable), especially in a drifter-float-fest event. Frankly, its a pretty marginal deal, unless you are racing a lot, and then, layline and execution,much more important to overall performance-still learning this myself....

Finally, the boats where booms make a huge difference are the monos. My prior experience was as a mate on a 70 ton displacement gaff-rigged schooner and some sloops. On mono hulls, especially schooners (no keel, only a skeg) the forces on the main and the shape of the main are a bit more critical to overall performance, and the forces on the boom are huge. A fast cat sail, by comparison to a mono tis overall, much flatter than a mono. On a cat, a fat round shape may accelerate the boat, but once up to speed, it is getting in the way.