I tend to think that this will be a tough sell. Yes, FOX 25 gets good ratings when they are the only game in town (as aptly stated above). And, they have had a knack for producing newscasts at convenient times which help to produce those ratings. But, 11 PM is a different matter. There is already an established audience for 11:00 newscasts and that audience tends to stick with the old-line Big 3 channels. Do you really think that a significant audience will be pried away from WCVB to watch FOX 25 @ 11? Not happening. Same is true of 7News and even WBZ. They have their audiences and, frankly, they're stronger looking newscasts.
The folks who watch a 10 PM newscast tend to do so because they want to check out the news and then go to bed. They won't hang around in droves to watch the same stuff at 11. And, now FOX loses the audience that wanted to see something instead of news at 11 (e.g. Seinfeld, Simpsons, etc.).
So, my prediction: FOX 25 News @ 11 will end up in a solid 4th place. And will stay there.
Management at FOX Boston will learn a hard lesson that stations in other markets have learned: it can be much more profitable to counter-program with Seinfeld reruns at 11 than it is to spend all that money on an extra hour of news that actually garners lower ratings and not as stellar demos.
FOX 25's niche has been providing a newscast at convenient times when no one else is (10 pm, mornings after 7). They should concentrate on that effort, which has seen success. Look at how they clobber 7 News on 56. But, they will get clobbered by 7 News at 11. Different audience, different level of competition.