I'm about ready to graduate to an HP handle too. I can't help but believe the cables are flexing enough to affect my shapes now that I can fly some of them consistently well. Consistently enough that I expect to be able to see the difference. I use the unbiased Tom Morris cable handles now, so I can probably just swap bars.
I used an old yellow plastic-gripped Sullivan as a beginner, and the cables began to fray, so I quit using it. But I know those kinky old cables affected my shapes. I was flying a Goldberg shoestring with a too-small control horn and the plane was super twitchy, but I didn't know any better at the time. It reminds me of flying a 1/2A. I can't even fly a decent pattern on it now, it's no wonder I was winding up loops the size of a beachball Lol. Then I started buying the tall steel Sullivan horns to tame it down a bit, and they don't flex like the nylon ones.
Rusty