My brother did this modification on his Catalina 380. It worked out very well. One of the benefits of having the boom higher is that it allows the dodger to be higher. Getting in and out of the cabin can be uncomfortable if the dodger is low - so he really likes this modification. The boat looks great, not at all "top heavy", in my opinion. This is, of course, a very individual and subjective opinion. So, how did he do it?
The goose neck was taken off. A new "collar" - with gooseneck - was fabricated that wrapped part-way around the mast at the desired location. It was attached and the boom then attached to it. The sail was recut with a bit of a "rounded" head section. When you look at the sail it just looks regular. The boat sails great, and there might be some loss of downwind speed because of the (slightly) less sail area, but I can't tell.
The spar work was done at Svenson's yard in Alameda. The sail was recut at Doyle sails, also in Alameda. Anyone familiar with the SF Bay Area knows that these are two of the most respected shops around. So you can be sure the job is "ship-shape and Bristol fashion". It was a great modification. Hope this helps.