When there's no chance of the antenna being damaged, I fit a Diamond SRH 320A (14") to my VX-6R. It has noticeable improvements on 2m and 440 but also in receiving the air band. However, it is more fragile than the stock resistor.
As for 6m, I wouldn't expect much from any
HT antenna.... the SRH999 is quad band and should yield similar performance (2m and up) as the 320A but comes it at 19.5".
I can't imagine walking around with a 20" antenna slapping me in the face with the radio strapped to the chest pack.