I agree that Sierra is very good in a Connie if you can find one - I couldn't.
Generally the very best soundcard at a price is the Phoenix, I have a couple but not in Connies. These are light enough to consider buying in USA, I used RLD.
Have just installed the Chuff cam version of MyLocoSound in a Connie. The whistle is not very prototypical but with twiddling can be made to sound fun. I turn the bell off, that's the worst bit. The steam hiss and chuff are very good though and its good and loud when installed with a decent speaker. Generally OK for a Connie at a reasonable price.
Hope this helps
Mike