This actually sounds quite interesting. You're starting from a good language (Smalltalk) and adding a feature absent from it but widely regarded as desirable (strong static typing).
> and clean up its syntax a bit (it's already pretty clean, just a few things could be improved)
If you go for type inference instead of having types explicitly declared, you might be able to avoid both run-time type checks and changes to Smalltalk's syntax.
> essentially a faster Smalltalk.
Bryce Kampjes (http://www.kampjes.demon.co.uk/index.html) was working on compiling Smalltalk byte code into machine code to speed it up, but that didn't involve changes to the language, so I think run-time type checks would still have been necessary. Making static type checks would result in a further speed-up.
Good luck.