No argument on the peak torque, but to me the big difference is WHERE the torque comes in on the power curve... The curves I've seen seem to say that the crotch rockets don't put out a lot of power until you have really cranked up the RPMs - no problem w/ a light weight bike to get it up to the speed where the HP kicks in, but w/ a heavy sidecar rig it means slipping the clutch like mad or otherwise abusing the bike to get it moving...
With the lower peak power "grunt" engines, the curves I have seen, seems to be that you have more power coming out just as you come off idle, plus big heavy flywheels that give plenty of oomph to get things moving along...
But this is the point. If you overlay the curves of say a harley, with a 80s 1100 suzuki the jap bike actually has more torque at all
Occasionally the harley will have a fraction more at really low rpm. Where you dont drive. It doesent feel that way because the harley makes less still as revs go up making the bottom feel good. But the suzuki gets even better as revs increase making it feel weak at the bottom. Its mostly an illusion. And why stuff like jap bike engined cars are becoming common place.
Torque plotted against road speed rather than rpm is even better x about 2. Why? Because the faster spinning jap bike has shorter gearing. Increasing torque at the wheels at all engine rpms by double at comparable road speeds. My dyno software is old now and wont run on modern computers or I would print out or screen grab some eg for you.
Jab bike idles at 800 rpm at revs to 10k or more. Harley same idle and 5k at best. So needs 2x different gearing for same road speed. Which means that the jap bike rpm range should really start at 2k to be compared in rpm range too. Its the same thing as the harley starting at 2k rpm... It really should be idling at sub 400 revs. But it cant. So idle torque advantage isnt "real", mostly its an illusion.
In the same way people think deisels have better pulling power. But they actually just have limited RPM range and little power or flexibility. Hence about 8 gears to row it along.