Even more effective is shedding pounds from the rider....
I met a guy last weekend, about 6' tall, maybe 220 pounds, who used to race 250cc street bikes. He said there was this woman rider, who weighed about 100 pounds less, who would wave at him as she passed him on the same model motorcycle.
Yes. It seems pretty obvious that power-to-weight ratio
must include the rider. This would obviously affect acceleration, and it seems quite logical that "top speed" might also be affected by greater wind resistance and road resistance. I've only just started riding my XT225, but I can tell you there is a very obvious difference in performance between taking my daughter (81 lbs) and taking my wife (125 lbs) as passengers. Really, very noticeable.
Especially when fighting gravity.
Whether your rule of thumb is accurate or not, I could surely stand to lose 21 pounds. If I gain the equivalent of 3 HP in the process, great. At least my bike won't sound obnoxious.
If your rule of thumb is wrong, then hey, I'm still healthier.
