Nearly every one of the top combat pilots in B class use the MAS 9x4 prop.
The MAS props are generally stronger than other brands, which is important in combat.
The 9x4 is used because straight line speed is not that important, however, maintaining speed in constant high-G turns is critical. A 9x4 allows the engine to continue to pull hard though the turns. If you run a 9x5 or 9x6, you'll go fast in a straight line, but once you start to turn or loop, you'll see the speed bleed off, and you won't be able to continue the turn.
Tips for getting cuts:
As Bob said, pick a target, and keep on them until you get them.
Combat is a 3-dimensional game. It's very hard to judge all three at a time, you try to position your plane relitive to your targets such that you can eliminate one or two dimentions.
AJ did a great writeup here:
http://www.rccombat.com/sections/workshop/workshop.asp
click on TEAMseaholm on the left side, then on "Aerial Tactics and Techniques" on that page.
I have 3 or 4 "setups" I like to use to get a cut on someone. These are relationships between my plane and my target that I look for that I can comfortably use to run my plane across the tail of my target, taking their streamer. AJ's article describes one of the best, I have a few others I like as well.
One of the absolute key elements to being good at combat is learning how to not watch your own plane too much. You have to look away from your own plane in the air to follow your target. If you concentrate on your own plane in the air, you won't be able to see and follow someone else, so you have to "soften" or "expand" your vision and fly that way.
Know your plane. If you aren't comfortable with your plane, you won't be able to look away from it in the heat of combat.
If possible know your opponents. I like to get up in the air as soon as I can in the 90second window to make sure I'm in trim, but to also size up my opponets. I'm looking for types of planes that I'm familiar with, looking for particular pilots, and looking for planes that seem to be faster or slower than mine, so I know what kind of approach I have to use to get them.