I do not know who is claiming that big wheels in the rear of the suspension will increase your track speed but they are wrong. Those wheels have nothing to do with the turning speed of the track.
It is the cogs on the drive shaft itself along with the gearing between the jackshaft and driveshaft that determines the speed of the track in relation to the gearing of the clutches themselves.
To get a longer lugged track in older sleds one usually has to drop the drive shafts cog size down. As an example on my 99 ZR700 in order to get a 144 inch track with 2 inch lugs I had to drop from 9 sprocket cogs to 8 sprocket cogs on my drive shaft to keep the track from hitting the cooler in the front of the tunnel. I also had to lower the mount point on the rear portion of the suspension in order to keep it from hitting the cooler that ran along the tunnel and make the suspension actually ride able.
The changing of the cogs from 9 to 8 caused a overall reduction in top end speed but gives me more power outta the hole and all through the throttle. Its all in the gearing ratio. Now yes I could switch out those gears in the chain case and be back to the top end speed I was but I would loose my power through out the throttle and that would mean at top end I would have less power to climb with.
You could port your track and that would give you a speed increase as what you are doing is cutting holes in your track making it lighter and allowing more air to travel through your track causing less drag on your engine.
An the Big Wheels they help keep the track from stabbing into the rear of your rails as you go over buried objects like rocks and stumps. The use of only the inside wheels helps the track flex a bit so that when you shift your entire body weight from one side of the sled to the other it helps you carve those turns.
Hope that helps.