I didn't notice much of a difference when going from v 8 to v 8.5 (which was at least a few months ago), and it was a free minor version update, so I don't think there are any earth-shattering changes. O&O Defrag 8 in general however, if you have not used it, is absolutely outstanding!
I think the most likely back-end changes to any defrag software are optimizations to any number of its system-intensive functions, such as the various types of analysis and data layout options, as well as the defragmentation process itself. Most of the time such changes go unnoticed by casual users, and a lot of times UI changes are the ones that stand out as a result.
I think in order to be worth paying for an upgrade to defrag software should either introduce new features, or provide a substantial improvement to the usability or performance.
One feature I'd love to see in a future version of O&O is a graphical layout builder where you can arrange the drive's data and then carry out a defrag based on the generated file. Currently you can use a layout file but must create it manually and there isn't abundant information about the steps required to do so. If that's a part of the next version, it'll be an upgrade I'll get for sure.
Regarding O&O vs PerfectDisk:
I tested O&O and PerfectDisk a year or so ago, and thought they both were about equal as far as features and performance. After having used O&O ever since, I recently downloaded the new PerfectDisk evaluation and tested it out. I was surprised to find the interface was noticeably laggier than O&O, didn't look quite as polished, and it took almost twice as long to analyze my drives to boot! At that point my PD re-evaluation was over and my faith in O&O reaffirmed.