Anton, I own a twinpower SW with both 5000 and 6000 spools (same gear ratio 6.2:1).
The 5000 spool measures 53mm across the lip, whilst the 6000 measures 56 mm. If you could fill the spool to the lip (which you cannot) the 6000 would retrieve 109 cm per crank and the 5000 103 cm per crank (using a little bit of Y2 maths). That's what the numbers say!
In reality, like you, I thought the 5000 was a little slow (its probably retreiving more like 95 cm) and reach for the 6000 spool whenever I require a little speed. The difference is noticeable, but as the numbers suggest - not huge.
If you are working your lure "walk the dog" style and using a long rod (8' plus) the 6000 spool does deliver a discernible advantage. And it does deliver a noticeable increase in casting distance.
At worst, having an extra spool allows for a spare pre-tied FG knot........just in case, you happen to have a bust off on the other!
Tight Lines