I've been wanting this since the mid-'70s. So, trying to research this, pull all the data into one posting, and counting gear teeth 'till my eyes are bleeding.
Here's what I've found so far in 'high ratio' primary gearsets:
Pic #1 - Australian Ivan Hoey's 32/69 gearset, copy of Tony Hall's Halco design. With roller bearings.
From the XS650 Australian Club website:
http://www.xs650.org.au/Racers.htm
Pic #2 - Comparison of stock 27/72 primary gearset to the 32/69 gearset.
Pic supplied by 270Yam.
Pic #3 - Heiden Tuning's offering, advertising a 35/80 gearset, but the picture shows a 32/69 gearset?
From Heiden's website:
http://www.xs650.biz/p/36/mo74-cg|29=1
Pic #4 - Riksoto's primary, shows a 32/68 gearset. A different geartooth combination. Hard to see, but looks like maybe a slightly larger diametric pitch is used?
Pic supplied by member Hamos (post #15).
Pic #5 - Required clearance grinding of the right side engine cover, to clear the larger engine gear.
From the XS650 Australian Club website:
http://www.xs650.org.au/Racers.htm
Gear ratios are usually referred to as input/output speed reduction ratios, but that creates an inconvenient number, so street convention for vehicles usually inverts the ratio, makes it easier to understand, especially if you're more concerned about torque multiplication (for drag racing) than output speed.
Using these primary gearsets teeth counts, sorted in increasing speed ratios, we get ratios of:
Stock --- 27:72 - speed ratio = 0.375, torque ratio = 2.667:1
Heiden - 35:80 - speed ratio = 0.438, torque ratio = 2.286:1
Hoey --- 32:69 - speed ratio = 0.464, torque ratio = 2.156:1
Riksoto - 32:68 - speed ratio = 0.471, torque ratio = 2.125:1
These sprocket combinations would be used to return to original rpm/speed ratio:
Stock --- 17 / 34
Heiden - 17 / 40
Hoey --- 17 / 42
Riksoto - 17 / 43