You will actually gain speed by installing bigger tires and loose speed with smaller tires...BUT its weight dependant. Most people either don't believe this or overlook it because in most cases, the bigger the tires...the heavier they are (resulting in speed loss) and smaller tires are more lightweight which will usually gain speed up to a certain point if they're lighter than other "bigger" tires. If you could find a very lightweight tire, let's say like a 24" that weigh the same as your 20" tires you will gain a little speed because of the increased rolling distance.
As far as the sprockets go, if I understand correctly, you are wanting to gear your trike for pure top end speed. In that case you will want the biggest front sprocket and the smallest rear sprocket available.
Justin ~hondaATCman~
'82 Honda ATC 110
'81 Honda ATC 250r (Summer Project)
'87 Suzuki LT 250r Quadracer - Very Modded...FAST!!
'03 Yamaha Big Bear 400 4x4
'85 Ford Bronco II 4x4 2.8 V6