C1/C2/PIE/PIF/Beta/Jitter/Whatever else are mostly theoretic scans. There is too much uncertainty when judging results, eg 1 single spike of 5 PIF's could in theory result in PO (don't quote me on that though), while 20+ could still be readable. It's only useful for relative comparison, eg a burn with "a lot of" PIE errors is "more likely" to result IN POF errors (unreadable data), but it depends on how those errors appear and many more factors.
Speed scan is "practical" judgement. If your drive manages to read the written disc without any problems (dips), then it means it didn't have any problems reading the disc. You should try this at highest speed possible, because the higher the speed, the less errors are needed to cause disruption. Furthermore, even if you DO have dips, they dont mean your burn is bad burn, though if you try it on standalone it might have more problems reading it at needed speed.
Whats even worse, is that you cant directly relate amount of PIE/PIF/POE errors to longevity of disc.. while it is true that "more" of these errors "could" mean that disc will become unreadable "sooner", it also depends on the dye used, protective, reflective layer and so on so forth.
So don't forget: take all these scans with a grain of salt. Never get scared about "too many" errors (gosh, you DEFINITELY dont need to get scared about few 5-20 PIF spikes here or there). So if I would have to rate scans in order of importance, it would go like:
Data verification (POF) > Read speed scan > C1/C2/PIE > POE/PIF > Jitter, beta, etc etc..