Any extensions that are compatible with DW CS5, should not have any issues in DW CS5.5
Not to split hairs, but not all extensions that are compatible with DW CS4 will be compatible with DW CS5 or even DW CS5.5.
If you post the names and version numbers of the extensions you have, I will be able to tell you if they are compatible or not.
Did you read Dave's second sentence
Your post is not specific on whether you are asking about WebAssist extensions or general extensions. Dave's first sentence was addressing general extensions. his second sentence addresses WebAssist Extensions.