I think 15/42 will be too tall. It really depends on your intended use though.
If you ride tight twisties, or dirt or race on a kart track it is way too tall.
I had my 550 in sumo trim, I ran it 14/45 for a short time. It was really fun, and could get all my shifting done quickly, but it was screaming at 90 mph.
I really liked 15/45, it seemed near perfect.
My 550 revs nearly as high as your 450 does, and hit 100 easily, with that gearing.
It cruised nicely at 70-80 mph in top gear, and didn't have a big gap between 5th and 6th.
That's the biggest problem you will have running too tall of a ratio, the gap between 5th and 6th, as 6th is an overdrive.
It will force you to either go faster in 6th or slower in 5th. Or lug/rev it. It can be an issue at slower speeds also. For twisty tight 30-50 mph type rides, The gaps between the lower gears can be annoying, making you rev it out into the next corner. It's either wind it out or shifting up and down alot.
I think 15/42 will give you that gap issue. You might not be able to cruise at say 62 mph. It will either be lugging in 6th or revving high in 5th.
I figure if you already have a 44, you could just pick up a 15t and try it out.