you wont get 20kph out of 6 teeth change (4 back and 1 on the front is the same as subtracting 2 from the back or close to anyway) you may gain 5 - 8kph, nothing more, they dont make a huge difference, you will probably gain about 15% which to my calculations would be something like 12kph gain but in real terms, unless you want to nail it for a minute or so to really stretch out the top end, its not worth the small gain.
i hit about 75 - 80kph, not too sure but i know it would be close to that, we had a trike which went 62kph and my bike was flogging it on the straight so thats my guess,, 75kph. i am running 17/41 with 14"12" rims. cant wait to put my street tyres on and see how she fairs with the installed irk.
Cheers, Tony.