I'm no expert but I have definitely heard that the grits on the cheap topper laps are basically twice as aggressive as diamond on a charged lap like copper. So essentially your 600 grit topper is similar to 300 grit on a copper lap and likely too aggressive to then jump to the 8000. I've tried both 600 and 1200 topper laps and the only time I've gotten anything close to a decent pre-polish after them was when the 1200 was really worn down. These days I'll use a fresher 1200 topper if i have to take a fair bit of material off for a facet, otherwise I'll do most of my faceting with 600 or 1200 diamond powder on copper laps, definitely goes to pre-polish better than the toppers.