we're currently rebuilding a client's application and are wondering if there are any benefits of serving specific pages to crawlers?
I.E. Semantic html, no stylesheets, additional meta tags?
Would love to know what you think
That would be a disaster and Google warns you never to do that. Serving pages that are different than what a user would view is considered black hat and will get you banned and put on wanted posters in most countries.
I couldn't quickly find Google's statement on this so I'll leave that as an exercise for you.