I too moved "up" the Ortofon MM tree from FF15E to OM10 to 520 Mk2 and later 530 Mk2 (all essentially the same working bits). I found the 520 a significant improvement to the OM10 in that it tracked a little better and was distinctly more lively and involving - good fun if you like. The 530 I found much more natural and realistic, clear, and a great improvement in tracking ability. As much "fun" as the 520? Possibly not but definitely a better presentation of the music. Horses for courses and different rather than better, as has been pointed out. Not an expensive upgrade path either. As for the 540/OM40/2m Black? Well, I look forward to reaching that top rung some day.
Ron
Main: Michell Tecnodec Rega RB600/Tecno bits, Ortofon 530MkII, Beard P100 phono, Naim Uniti Atom, ProAc D20R
Study: JBL L52 Classic, Bluesound Powernode 3, Technics SL-1210GR Denon 110, Project Tube Box S2 phono stage
Other stuff: Technics SL1200 Mk2, Dali Spektor 1, Pro-ject MAIA S3 etc.