Since SpellingService is using your system dictionary and spellchecker, I imagine the quality of the spelling suggestions should be close to optimal, without actually diving into that area of research ourselves ;)
I integrated SpellingServices (http://www.hpi.uni-potsdam.de/hirschfeld/squeaksource/SpellingServices) and Refactoring-Spelling (the spell checker for the refactoring browser). The changes are in the attached monticello file.
if SpellingServices is installed in your image then the spell checker in the refactoring browser will be: (in order of preference) 1. The RBMacSpellChecker 2. The RBSpellingService 3. The RBInternalSpellChecker
I only tested it on 64bit Linux. It runs much slower than the RBInternalSpellChecker. But it catches more errors. Or just has a dumber vocabulary. I can't tell yet. All tests pass.