Backbone has limitations. Its slow hence the 020 requirement.
I'm surprised that requiring an 68020 makes anything significantly faster. There are instructions in the 68020 that are expensive to emulate exactly with 68000 compatible code, but unless you really need that exact semantics in the innermost loop there is little reason to actually use them. 20 something years ago I backported a C64 emulator (can't remember the name; Frodo?) from 68020 to 68000 (I only had an A2000). The result used less cycles on an 68020 than the original code (counting cycles was easy with these - by todays standards - primitive CPU architectures). That doesn't mean it wouldn't have been possible to create an even more optimized 68020 only version, but usually it is - easily, I'm definitely no Notaz or Exophase - possible to restructure your code to be 68000 compatible without a noticable loss of performance.