HPC has certainly helped my observing, not least because there are some fantastic coaches (both professional and not) in the Club and their feedback on my own driving has given me a lot that I can recycle on other people. It also means that, when I come to do a demonstration drive, I can be pretty confident that I will drive consistently above the standard required for IAM or RoADA, even though I'm still a baby in HPC terms.
The other thing that helped my observing a lot was reading
Coaching for Performance by Sir John Whitmore. Highly Recommended, and available cheaply from Amazon!
James
IAM Observer, RoADA Gold & Tutor, HPC muppet
Only two things matter: attitude & entry speeds.