There was a good article regarding polyurethanes ,I can't find it but the jest is as the poly's cure the good stuff rises to the surface in a very thin, very hard layer. If you sand through this (and you will) you open the softer undercoat to the elements. Like Jake said if you get it right then it's fine as long as you don't plan on doing spot repairs. I remember when we built Kialoa IV the painter orange peeled a good portion of the hull using Awlgrip. The only way to fix was sand it smooth and spray it again. A lot of work and a lot of money on an 80ft hull. So in the future if you want to do small repairs to a poly paint you'll have to re coat the whole boat or at least the side to have a perfect finish.

I used to do small paint repair for car dealerships. I would say most of the cars I did spot work on wouldn't halo. Almost always you would see it on darker colors without flake or pearl like Black or Jaguar British Racing Green. The halo would appear when using the clear coat thinner to blend the edges. I believe since I did most of my work outside in the Florida sun part of the cause was the amount of thinner I would spray trying to blend the edge because the heat was curing the paint so fast. I can't remember ever seeing it happen on a white car. Now a days there are blending solvents that will not halo.

The advantage of Gelcoat is, it's so easy to repair.

There are single stage (no clear coat) urethanes you might want to check out. Never use anything without a catalyst.