My bikes have headtube lengths varing from 2 .5" to 7.5". But I always associate headtube length with structural issues not handling. A long headtube can make a frame more trapezoidal than triangular and rigid. This can put a lot of strain on the downtube.
High top tube height is bad for steep hill climbing because it can make speedy bale outs tricky.
One thing in this area is stem length and offset. I have some adjustable stems (0 to 100mm offset / 0 to 90 degree angle). Changes can have marked effects on low speed and hill climbing stability.
I am still refining my thoughts on effects of COG positioning and balance. And hope to produce some interesting conclusions soon.